Mobile communication terminal device and method for selecting a virtual carrier for machine-type communications based on measurements of channel conditions

ABSTRACT

An apparatus and method providing feedback on channel conditions in a wireless telecommunications system including a base station to communicate with plural terminals device using frequencies spanning a system frequency bandwidth. At least one terminal device is a reduced capability terminal device including a tuneable transceiver configured to receive downlink transmissions from the base station using only a restricted frequency bandwidth smaller than and within the system frequency bandwidth. The reduced capability terminal device is configured to communicate information derived from measurements of channel conditions to the base station. The information may include an indication of measured channel conditions for different frequency locations, or an indication of one or more frequency locations for which corresponding measurement of channel conditions meet a pre-defined selection criterion. The base station subsequently schedules downlink transmissions for the terminal device in a manner that takes account of the information received from the terminal device.

BACKGROUND OF THE INVENTION

The present invention relates to methods and apparatus for use inwireless (mobile) telecommunications systems. In particular, embodimentsof the invention relate to methods and apparatus for reporting onchannel conditions in wireless telecommunications systems.

Third and fourth generation mobile telecommunication systems, such asthose based on the 3GPP defined UMTS and Long Term Evolution (LTE)architecture, are able to support more sophisticated services thansimple voice and messaging services offered by previous generations ofmobile telecommunication systems.

For example, with the improved radio interface and enhanced data ratesprovided by LTE systems, a user is able to enjoy high data rateapplications such as mobile video streaming and mobile videoconferencing that would previously only have been available via a fixedline data connection. The demand to deploy third and fourth generationnetworks is therefore strong and the coverage area of these networks,i.e. locations where access to the networks is possible, is expected toincrease rapidly.

The anticipated widespread deployment of third and fourth generationnetworks has led to the parallel development of a class of devices andapplications which, rather than taking advantage of the high data ratesavailable, instead take advantage of the robust radio interface andincreasing ubiquity of the coverage area. Examples include so-calledmachine type communication (MTC) applications, which are typified bysemi-autonomous or autonomous wireless communication devices (i.e. MTCdevices) communicating small amounts of data on a relatively infrequentbasis. Examples include so-called smart meters which, for example, arelocated in a customer's house and periodically transmit information backto a central MTC server relating to the customers consumption of autility such as gas, water, electricity and so on. Further informationon characteristics of MTC-type devices can be found, for example, in thecorresponding standards, such as ETSI TS 122 368 V10.530 (2011-07)/3GPPTS 22.368 version 10.5.0 Release 10) [1]. Some typical characteristicsof MTC type terminal devices/MTC type data might include, for example,characteristics such as low mobility, high delay tolerance, small datatransmissions, infrequent transmission and group-based features,policing and addressing.

Whilst it can be convenient for a terminal such as an MTC type terminalto take advantage of the wide coverage area provided by a third orfourth generation mobile telecommunication network there are at presentdisadvantages. Unlike a conventional third or fourth generation terminaldevice such as a smartphone, an MTC-type terminal is preferablyrelatively simple and inexpensive and able to operate on relatively lowresources (e.g. low power consumption). The type of functions performedby the MTC-type terminal (e.g. collecting and reporting back data) donot require particularly complex processing to perform, and furthermoreare typically not time-critical. However, third and fourth generationmobile telecommunication networks typically employ advanced datamodulation techniques on the radio interface which can be power hungryand require more complex and expensive radio transceivers to implement.It is usually justified to include such complex transceivers in asmartphone as a smartphone will typically require a powerful processorto perform typical smartphone type functions. However, as indicatedabove, there is now a desire to use relatively inexpensive and lesscomplex devices able to operate with low resource usage to communicateusing LTE type networks. To this end, so-called “virtual carriers” havebeen proposed.

Some characteristics of virtual carriers are discussed in more detailfurther below. However, in brief summary, certain classes of devices,such as MTC devices, may support communication applications that arecharacterised by the transmission of small amounts of data at relativelyinfrequent intervals and can thus be considerably less complex thanconventional LTE devices. Typical LTE communications devices may includea high-performance receiver unit capable of receiving and processingdata from an LTE downlink frame across the full carrier bandwidth.However, such receiver units can be overly complex for a device whichonly needs to transmit or to receive small amounts of data. This maytherefore limit the practicality of a widespread deployment of reducedcapability MTC type devices in an LTE network. It has therefore beenproposed to provide reduced capability devices such as MTC devices witha simpler receiver unit which is more proportionate with the amount ofdata likely to be transmitted to the device. Furthermore, as explainedabove it is desirable to include features in a mobile communicationsnetwork and/or communications devices which can conserve powerconsumption of the communications devices.

In conventional mobile telecommunication networks, data is typicallytransmitted from the network to the communications devices in afrequency carrier (first frequency range) where at least part of thedata might span substantially the whole of the bandwidth of thefrequency carrier. Normally a communications device cannot operatewithin the network unless it can receive and decode data spanning theentire frequency carrier, i.e. a maximum system bandwidth defined by agiven telecommunication standard, and therefore the use ofcommunications devices with reduced bandwidth capability transceiverunits can in effect be precluded from operating with such a carrier.

However, in accordance with the previously proposed virtual carrierconcepts, a subset of the communications resource elements comprising aconventional carrier (a “host carrier”) are defined as a “virtualcarrier”, where the host carrier has a certain bandwidth (firstfrequency range) and where the virtual carrier has a reduced bandwidth(second frequency range) compared to the host carrier's bandwidth. Datafor reduced capability devices is separately transmitted on the virtualcarrier set of communications resource elements. Accordingly, datatransmitted on the virtual carrier can be received and decoded using areduced complexity or capability transceiver unit (i.e. one with atransceiver having a narrower operating bandwidth than would otherwisebe required to operate in the network).

Devices provided with reduced complexity or capability transceiver units(hereafter referred to as “reduced capability devices”) could operate byusing a part of its full capability (i.e. reduced capability set of itsfull capability) or they could be constructed to be less complex andless expensive than conventional LTE type devices (onwards referred togenerally as legacy LTE devices). Accordingly, the deployment of suchdevices for MTC type applications within an LTE type network can becomemore attractive because the provision of the virtual carrier allowscommunications devices with less expensive and less complex transceiverunits to be used.

Conventional LTE type networks allow for so-called link adaptation by ascheduler of a base station. Link adaptation allows a base station tomodify its transmissions characteristics in a manner which takes accountof channel conditions existing between the base station and a terminaldevice. For example, higher data rates may be used when channelconditions compared to when channel conditions are bad. A significantaspect of link adaptation is channel quality indicator (CQI) reporting.As is well established, a terminal device may measure the channelquality of a downlink communication and report it back to the basestation as a CQI report. The base station may then perform linkadaptation based on the CQI report.

Existing LTE standards provide for CQI reports with two types ofbandwidth. One is known as wideband CQI and the other is known assub-band CQI. For wideband CQI a single CQI value is established for acarrier's full bandwidth and reported to the base station. For sub-bandCQI, the full bandwidth is in effect split into more than one sub-band,and a CQI value is established for each sub-band. The wideband CQIapproach is simple and provides for compact signalling whereas thesub-band CQI approach can allow a scheduler to take account of frequencyselective channel conditions (e.g. frequency-dependent fading).

The inventors have recognised that particular considerations might applywhen considering channel conditions, for example through CQI measurementand reporting, in the context of virtual carriers. In principle aterminal device operating on a virtual carrier can implement CQImeasurement and reporting within the virtual carrier in accordance withthe same principles as used for conventional CQI measurement andreporting within a conventional carrier. However, in accordance withvirtual carrier techniques there is in principle the possibility of abase station scheduler moving a virtual carrier from one frequency bandto another, for example because the existing virtual carrier frequencyband is subject to poor channel conditions. However, there is currentlyno mechanism for providing a base station scheduler with information toallow the base station scheduler to determine whether or not it would beappropriate to move a virtual carrier from one frequency to another.

There is therefore a desire to provide for improved schemes forreporting on channel conditions in wireless telecommunications systems.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a methodof operating a terminal device in a wireless telecommunications systemcomprising a base station arranged to communicate with a plurality ofterminal devices using frequencies spanning a system frequencybandwidth, wherein the terminal device comprises a tuneable transceiverconfigured to receive downlink transmissions from the base station usinga restricted frequency bandwidth which is smaller than and within thesystem frequency bandwidth, and wherein the method comprises:sequentially tuning the transceiver to different frequency locations forthe restricted frequency bandwidth within the system frequency bandwidthand making measurements of channel conditions at the different frequencylocations to provide a corresponding plurality of measurements ofchannel conditions; and communicating information derived from theplurality of measurements of channel conditions to the base station.

In accordance with some embodiments the different frequency locationsfor the restricted frequency bandwidth within the system frequencybandwidth comprise a plurality of predefined frequency locations.

In accordance with some embodiments the measurements of channelconditions comprise measurements of noise and/or measurements ofinterference.

In accordance with some embodiments the information communicated to thebase station comprises an indication of at least a subset of theplurality of measurements of channel conditions.

Some embodiments further comprise the terminal device selecting apreferred frequency location for the restricted frequency bandwidthwithin the system frequency bandwidth based on the plurality ofmeasurements of channel conditions.

In accordance with some embodiments the information communicated to thebase station comprises an indication of the preferred frequency locationfor the restricted frequency bandwidth within the system frequencybandwidth.

In accordance with some embodiments the information communicated to thebase station further comprises an indication of a measurement of channelconditions for the preferred frequency location for the restrictedfrequency bandwidth within the system frequency bandwidth.

In accordance with some embodiments the information communicated to thebase station comprises an indication of locations for the restrictedfrequency bandwidth within the system frequency bandwidth associatedwith measurements of channel conditions meeting a predefined criterion.

In accordance with some embodiments the information communicated to thebase station comprises an indication of one or more times associatedwith the plurality of measurements of channel conditions.

In accordance with some embodiments the downlink radio frame structurefor the wireless telecommunications system comprises a series of timeslots and the measurements of channel conditions for different frequencylocations are made in different time slots.

In accordance with some embodiments consecutive measurements of channelconditions for different frequency locations are made in non-consecutivetime slots.

In accordance with some embodiments the information derived from theplurality of measurements of channel conditions is communicated to thebase station with signalling on a layer which is higher than a physicallayer.

In accordance with some embodiments the information derived from theplurality of measurements of channel conditions is communicated to thebase station with radio resource control, RRC, signalling.

In accordance with some embodiments the information derived from theplurality of measurements of channel conditions is communicated to thebase station with physical layer signalling.

In accordance with some embodiments the information communicated to thebase station is sent in a plurality of channel condition reportscorresponding with respective one of the plurality of measurements ofchannel conditions.

Some embodiments further comprise receiving a plurality of resourceallocation messages from the base station indicating uplink transmissionresources to be used for sending respective ones of the channelcondition reports.

In accordance with some embodiments different ones of the plurality ofresource allocation messages are received with the transceiver tuned todifferent frequency locations for the restricted frequency bandwidthwithin the system frequency bandwidth.

Some embodiments further comprise receiving from the base station anindication of the different frequency locations for the restrictedfrequency bandwidth within the system frequency bandwidth at which tomake the plurality of measurements of channel conditions

Some embodiments further comprise receiving an indication from the basestation that the terminal device's transceiver should be retuned to afrequency location selected by the base station based on the informationreceived from the terminal device.

According to a second aspect of the invention there is provided aterminal device for use in a wireless telecommunications systemcomprising a base station arranged to communicate with a plurality ofterminal devices using frequencies spanning a system frequencybandwidth, wherein the terminal device comprises a tuneable transceiverconfigured to receive downlink transmissions from the base station usinga restricted frequency bandwidth which is smaller than and within thesystem frequency bandwidth, and wherein the terminal device isconfigured to: sequentially tune the transceiver to different frequencylocations for the restricted frequency bandwidth within the systemfrequency bandwidth and make measurements of channel conditions at thedifferent frequency locations to provide a corresponding plurality ofmeasurements of channel conditions; and to communicate informationderived from the plurality of measurements of channel conditions to thebase station.

According to a third aspect of the invention there is provided a methodof operating a base station in a wireless telecommunications systemcomprising a base station arranged to communicate with a plurality ofterminal devices using frequencies spanning a system frequencybandwidth, wherein the plurality of terminal devices includes a reducedcapability terminal device comprising a tuneable transceiver configuredto receive downlink transmissions from the base station using arestricted frequency bandwidth which is smaller than and within thesystem frequency bandwidth, and wherein the method comprises: receivingfrom the reduced capability terminal device information derived by froma plurality of measurements of channel conditions obtained by thereduced capability terminal device sequentially tuning its transceiverto different frequency locations for its restricted frequency bandwidthwithin the system frequency bandwidth and making measurements of channelconditions at the different frequency locations; and schedulingsubsequent downlink transmissions for the reduced capability terminaldevice on frequencies selected in a manner which takes account of theinformation received from the reduced capability terminal device.

In accordance with some embodiments the different frequency locationsfor the restricted frequency bandwidth within the system frequencybandwidth comprise a plurality of predefined frequency locations.

In accordance with some embodiments the measurements of channelconditions comprise measurements of noise and/or measurements ofinterference.

In accordance with some embodiments the information received from thereduced capability terminal device comprises an indication of at least asubset of the plurality of measurements of channel conditions.

In accordance with some embodiments the information received from thereduced capability terminal device comprises an indication of apreferred frequency location for the restricted frequency bandwidthselected from within the system frequency bandwidth by the reducedcapability terminal device in a manner which takes account of theplurality of measurements of channel conditions.

In accordance with some embodiments the information received from thereduced capability terminal device further comprises an indication of ameasurement of channel conditions for the preferred frequency location.

In accordance with some embodiments the information received from thereduced capability terminal device comprises an indication of locationsfor the restricted frequency bandwidth within the system frequencybandwidth associated with measurements of channel conditions meeting apredefined criterion.

In accordance with some embodiments the information received from thereduced capability terminal device comprises an indication of one ormore times associated with the plurality of measurements of channelconditions.

In accordance with some embodiments the downlink radio frame structurefor the wireless telecommunications system comprises a series of timeslots and the method further comprises the base station transmittingreference signalling at different times and frequencies to allow thereduced capability terminal device to make measurements of channelconditions for different frequency locations in different time slots.

Some embodiments further comprise the base station transmittingreference signalling at different times and frequencies to allow thereduced capability terminal device to make consecutive measurements ofchannel conditions for different frequency locations in non-consecutivetime slots.

In accordance with some embodiments the information is received from thereduced capability terminal device with signalling on a layer which ishigher than a physical layer.

In accordance with some embodiments the information is received from thereduced capability terminal device using radio resource control, RRC,signalling.

In accordance with some embodiments the information is received from thereduced capability terminal device with physical layer signalling.

In accordance with some embodiments the information is received from thereduced capability terminal device in a plurality of channel conditionreports corresponding with respective one of the plurality ofmeasurements of channel conditions.

Some embodiments further comprise the base station transmitting to theterminal device a plurality of resource allocation messages indicatinguplink transmission resources to be by the reduced capability terminaldevice for sending respective ones of the channel condition reports.

In accordance with some embodiments different ones of the plurality ofresource allocation messages are transmitted at different frequencylocations for the reduced capability terminal device's restrictedfrequency bandwidth within the system frequency bandwidth.

Some embodiments further comprise transmitting to the reduced capabilityterminal device an indication that the reduced capability terminaldevice should proceed to make the measurements of channel conditions atthe different frequency locations.

Some embodiments further comprise the base station transmitting anindication of the different frequency locations for the restrictedfrequency bandwidth within the system frequency bandwidth at which thereduced capability terminal device should make the plurality ofmeasurements of channel conditions

Some embodiments further comprise the base station transmitting anindication to the reduced capability terminal device to indicate thereduced capability terminal device should retune its transceiver to afrequency location corresponding to the frequencies selected for thesubsequent downlink transmissions.

According to a fourth aspect of the invention there is provided a basestation for use in a wireless telecommunications system comprising abase station arranged to communicate with a plurality of terminaldevices using frequencies spanning a system frequency bandwidth, whereinthe plurality of terminal devices includes a reduced capability terminaldevice comprising a tuneable transceiver configured to receive downlinktransmissions from the base station using a restricted frequencybandwidth which is smaller than and within the system frequencybandwidth, and wherein the base station is configured to: receiving fromthe reduced capability terminal device information derived by from aplurality of measurements of channel conditions obtained by the reducedcapability terminal device sequentially tuning its transceiver todifferent frequency locations for its restricted frequency bandwidthwithin the system frequency bandwidth and making measurements of channelconditions at the different frequency locations; and to schedulesubsequent downlink transmissions for the reduced capability terminaldevice on frequencies selected in a manner which takes account of theinformation received from the reduced capability terminal device.

It will be appreciated that features and aspects of the inventiondescribed above in relation to the first and other aspects of theinvention are equally applicable to, and may be combined with,embodiments of the invention according to other aspects of the inventionas appropriate, and not just in the specific combinations describedabove.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings where likeparts are provided with corresponding reference numerals and in which:

FIG. 1 provides a schematic diagram illustrating an example of aconventional mobile telecommunication network;

FIG. 2 provides a schematic diagram illustrating a conventional LTEradio frame;

FIG. 3 provides a schematic diagram illustrating an example of aconventional LTE downlink radio subframe;

FIG. 4 provides a schematic diagram illustrating a conventional LTE“camp-on” procedure;

FIG. 5 provides a schematic diagram illustrating an LTE downlink radiosubframe in which a virtual carrier has been inserted;

FIG. 6 provides a schematic diagram illustrating an adapted LTE“camp-on” procedure for camping on to a virtual carrier;

FIG. 7 provides a schematic diagram illustrating LTE downlink radiosubframes;

FIG. 8 provides a schematic diagram illustrating a physical broadcastchannel (PBCH);

FIG. 9 provides a schematic diagram illustrating an LTE downlink radiosubframe;

FIG. 10 provides a schematic diagram illustrating an LTE downlink radiosubframe in which a virtual carrier has been inserted;

FIGS. 11A to 11D provide schematic diagrams illustrating positioning oflocation signals within a LTE downlink subframe;

FIG. 12 provides a schematic diagram illustrating a group of subframesin which two virtual carriers change location within a host carrierband;

FIGS. 13A to 13C provide schematic diagrams illustrating LTE uplinksubframes in which an uplink virtual carrier has been inserted;

FIG. 14 provides a schematic diagram showing part of an adapted LTEmobile telecommunication network arranged in accordance with an exampleof the present invention;

FIG. 15 schematically shows a mobile telecommunication networkarchitecture according to an embodiment of the invention; and

FIGS. 16 and 17 are signalling ladder diagrams schematicallyrepresenting methods of operation in accordance with embodiments of theinvention.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Embodiments of the invention may in particular be employed within thecontext of what might be termed “virtual carriers” operating within abandwidth of a “host carriers”. The concepts of virtual carriers aredescribed in co-pending UK patent applications numbered GB 1101970.0[2], GB 1101981.7 [3], GB 1101966.8 [4], GB 1101983.3 [5], GB 1101853.8[6], GB 1101982.5 [7], GB 1101980.9 [8], GB 1101972.6 [9], GB 1121767.6[10] and GB 1121766.8 [11] the contents of which are incorporated hereinby reference. The reader is referred to these co-pending applicationsfor more details, but for ease of reference an overview of the conceptof virtual carriers is also provided here.

Conventional Network

FIG. 1 provides a schematic diagram illustrating some basicfunctionality of a wireless telecommunications network/system 100operating in accordance with LTE principles. Various elements of FIG. 1and their respective modes of operation are well-known and defined inthe relevant standards administered by the 3GPP (RTM) body and alsodescribed in many books on the subject, for example, Holma H. andToskala A [12].

The network includes a plurality of base stations 101 connected to acore network 102. Each base station provides a coverage area 103 (i.e. acell) within which data can be communicated to and from terminal devices104. Data is transmitted from base stations 101 to terminal devices 104within their respective coverage areas 103 via a radio downlink Data istransmitted from terminal devices 104 to the base stations 101 via aradio uplink The core network 102 routes data to and from the terminaldevices 104 via the respective base stations 101 and provides functionssuch as authentication, mobility management, charging and so on.

Mobile telecommunications systems such as those arranged in accordancewith the 3GPP defined Long Term Evolution (LTE) architecture use anorthogonal frequency division multiplex (OFDM) based interface for theradio downlink (so-called OFDMA) and the radio uplink (so-calledSC-FDMA). FIG. 2 shows a schematic diagram illustrating an OFDM basedLTE downlink radio frame 201. The LTE downlink radio frame istransmitted from an LTE base station (known as an enhanced Node B) andlasts 10 ms. The downlink radio frame comprises ten subframes, eachsubframe lasting 1 ms. A primary synchronisation signal (PSS) and asecondary synchronisation signal (SSS) are transmitted in the first andsixth subframes of the LTE frame. A primary broadcast channel (PBCH) istransmitted in the first subframe of the LTE frame. The PSS, SSS andPBCH are discussed in more detail below.

FIG. 3 is a schematic diagram of a grid which illustrates the structureof an example conventional downlink LTE subframe. The subframe comprisesa predetermined number of symbols which are transmitted over a 1 msperiod. Each symbol comprises a predetermined number of orthogonalsub-carriers distributed across the bandwidth of the downlink radiocarrier.

The example subframe shown in FIG. 3 comprises 14 symbols and 1200sub-carriers spread across a 20 MHz bandwidth. The smallest allocationof user data for transmission in LTE is a resource block comprisingtwelve sub-carriers transmitted over one slot (0.5 subframe). Forclarity, in FIG. 3, each individual resource element is not shown,instead each individual box in the subframe grid corresponds to twelvesub-carriers transmitted on one symbol.

FIG. 3 shows in hatching resource allocations for four LTE terminals340, 341, 342, 343. For example, the resource allocation 342 for a firstLTE terminal (UE 1) extends over five blocks of twelve sub-carriers(i.e. 60 sub-carriers), the resource allocation 343 for a second LTEterminal (UE2) extends over six blocks of twelve sub-carriers and so on.

Control channel data is transmitted in a control region 300 (indicatedby dotted-shading in FIG. 3) of the subframe comprising the first nsymbols of the subframe where n can vary between one and three symbolsfor channel bandwidths of 3 MHz or greater and where n can vary betweentwo and four symbols for channel bandwidths of 1.4 MHz. For the sake ofproviding a concrete example, the following description relates to hostcarriers with a channel bandwidth of 3 MHz or greater so the maximumvalue of n will be 3. The data transmitted in the control region 300includes data transmitted on the physical downlink control channel(PDCCH), the physical control format indicator channel (PCFICH) and thephysical HARQ indicator channel (PHICH).

PDCCH contains control data indicating which sub-carriers on whichsymbols of the subframe have been allocated to specific LTE terminals.Thus, the PDCCH data transmitted in the control region 300 of thesubframe shown in FIG. 3 would indicate that UE1 has been allocated theblock of resources identified by reference numeral 342, that UE2 hasbeen allocated the block of resources identified by reference numeral343, and so on.

PCFICH contains control data indicating the size of the control region(i.e. between one and three symbols).

PHICH contains HARQ (Hybrid Automatic Request) data indicating whetheror not previously transmitted uplink data has been successfully receivedby the network.

Symbols in a central band 310 of the time-frequency resource grid areused for the transmission of information including the primarysynchronisation signal (PSS), the secondary synchronisation signal (SSS)and the physical broadcast channel (PBCH). This central band 310 istypically 72 sub-carriers wide (corresponding to a transmissionbandwidth of 1.08 MHz). The PSS and SSS are synchronisation signals thatonce detected allow an LTE terminal device to achieve framesynchronisation and determine the cell identity of the enhanced Node Btransmitting the downlink signal. The PBCH carries information about thecell, comprising a master information block (MIB) that includesparameters that LTE terminals use to properly access the cell. Datatransmitted to individual LTE terminals on the physical downlink sharedchannel (PDSCH) can be transmitted in other resource elements of thesubframe. Further explanation of these channels is provided below.

FIG. 3 also shows a region of PDSCH containing system information andextending over a bandwidth of R₃₄₄. A conventional LTE frame will alsoinclude reference signals which are discussed further below but notshown in FIG. 3 in the interests of clarity.

The number of sub-carriers in an LTE channel can vary depending on theconfiguration of the transmission network. Typically this variation isfrom 72 sub carriers contained within a 1.4 MHz channel bandwidth to1200 sub-carriers contained within a 20 MHz channel bandwidth (asschematically shown in FIG. 3). As is known in the art, data transmittedon the PDCCH, PCFICH and PHICH is typically distributed on thesub-carriers across the entire bandwidth of the subframe to provide forfrequency diversity. Therefore a conventional LTE terminal must be ableto receive the entire channel bandwidth in order to receive and decodethe control region.

FIG. 4 illustrates an LTE “camp-on” process, that is, the processfollowed by a terminal so that it can decode downlink transmissionswhich are sent by a base station via a downlink channel. Using thisprocess, the terminal can identify the parts of the transmissions thatinclude system information for the cell and thus decode configurationinformation for the cell.

As can be seen in FIG. 4, in a conventional LTE camp-on procedure, theterminal first synchronizes with the base station (step 400) using thePSS and SSS in the centre band and then decodes the PBCH (step 401).Once the terminal has performed steps 400 and 401, it is synchronizedwith the base station.

For each subframe, the terminal then decodes the PCFICH which isdistributed across the entire bandwidth of carrier 320 (step 402). Asdiscussed above, an LTE downlink carrier can be up to 20 MHz wide (1200sub-carriers) and an LTE terminal therefore has to have the capabilityto receive and decode transmissions on a 20 MHz bandwidth in order todecode the PCFICH. At the PCFICH decoding stage, with a 20 MHz carrierband, the terminal operates at a much larger bandwidth (bandwidth ofR₃₂₀) than during steps 400 and 401 (bandwidth of R₃₁₀) relating tosynchronization and PBCH decoding.

The terminal then ascertains the PHICH locations (step 403) and decodesthe PDCCH (step 404), in particular for identifying system informationtransmissions and for identifying its resource allocations. The resourceallocations are used by the terminal to locate system information and tolocate its data in the PDSCH as well as to be informed of anytransmission resources it has been granted on PUSCH. Both systeminformation and UE-specific resource allocations are transmitted onPDSCH and scheduled within the carrier band 320. Steps 403 and 404 alsorequire the terminal to operate on the entire bandwidth R320 of thecarrier band.

At steps 402 to 404, the terminal decodes information contained in thecontrol region 300 of a subframe. As explained above, in LTE, the threecontrol channels mentioned above (PCFICH, PHICH and PDCCH) can be foundacross the control region 300 of the carrier where the control regionextends over the range R₃₂₀ and occupies the first one, two or threeOFDM symbols of each subframe as discussed above. In a subframe,typically the control channels do not use all the resource elementswithin the control region 300, but they are scattered across the entireregion, such that a LTE terminal has to be able to simultaneouslyreceive the entire control region 300 for decoding each of the threecontrol channels.

The terminal can then decode the PDSCH (step 405) which contains systeminformation or data transmitted for this terminal

As explained above, in an LTE subframe the PDSCH generally occupiesgroups of resource elements which are neither in the control region norin the resource elements occupied by PSS, SSS or PBCH. The data in theblocks of resource elements 340, 341, 342, 343 allocated to thedifferent mobile communication terminals (UEs) shown in FIG. 3 have asmaller bandwidth than the bandwidth of the entire carrier, although todecode these blocks a terminal first receives the PDCCH spread acrossthe frequency range R₃₂₀ to determine if the PDCCH indicates that aPDSCH resource is allocated to the UE and should be decoded. Once a UEhas received the entire subframe, it can then decode the PDSCH in therelevant frequency range (if any) indicated by the PDCCH. So forexample, UE 1 discussed above decodes the whole control region 300 andthen the data in the resource block 342.

Virtual Downlink Carrier

Certain classes of devices, such as MTC devices (e.g. semi-autonomous orautonomous wireless communication devices such as smart meters asdiscussed above), support communication applications that arecharacterised by the transmission of small amounts of data at relativelyinfrequent intervals and can thus be considerably less complex thanconventional LTE terminals. In many scenarios, providing low capabilityterminals such as those with a conventional high-performance LTEreceiver unit capable of receiving and processing data from an LTEdownlink frame across the full carrier bandwidth can be overly complexfor a device which only needs to communicate small amounts of data. Thismay therefore limit the practicality of a widespread deployment of lowcapability MTC type devices in an LTE network. It is preferable insteadto provide low capability terminals such as MTC devices with a simplerreceiver unit which is more proportionate with the amount of data likelyto be transmitted to the terminal. As set out below, in accordance withsome examples of the present invention a “virtual carrier” is providedwithin the transmission resources of a conventional OFDM type downlinkcarrier (i.e. a “host carrier”). Unlike data transmitted on aconventional OFDM type downlink carrier, data transmitted on the virtualcarrier can be received and decoded without needing to process the fullbandwidth of the downlink host OFDM carrier. Accordingly, datatransmitted on the virtual carrier can be received and decoded using areduced complexity receiver unit.

FIG. 5 provides a schematic diagram illustrating an LTE downlinksubframe which includes a virtual carrier inserted in a host carrier.

In keeping with a conventional LTE downlink subframe, the first nsymbols (n is three in FIG. 5) form the control region 300 which isreserved for the transmission of downlink control data such as datatransmitted on the PDCCH. However, as can be seen from FIG. 5, outsideof the control region 300 the LTE downlink subframe includes a group ofresource elements positioned in this example below the central band 310which form a virtual carrier 501. As explained further below, thevirtual carrier 501 is adapted so that data transmitted on the virtualcarrier 501 can be treated as logically distinct from data transmittedin the remaining parts of the host carrier and can be decoded withoutdecoding all the control data from the control region 300. Although FIG.5 shows the virtual carrier occupying frequency resources below thecentre band, in general the virtual carrier can occupy other frequencyresources, for example, above the centre band or including the centreband. If the virtual carrier is configured to overlap any resources usedby the PSS, SSS or PBCH of the host carrier, or any other signaltransmitted by the host carrier that a terminal device operating on thehost carrier would require for correct operation and expect to find in aknown pre-determined location, the signals on the virtual carrier can bearranged such that these aspects of the host carrier signal aremaintained.

As can be seen from FIG. 5, data transmitted on the virtual carrier 501is transmitted across a limited bandwidth. This might be any suitablebandwidth smaller than that of the host carrier. In the example shown inFIG. 5 the virtual carrier is transmitted across a bandwidth comprising12 blocks of 12 sub-carriers (i.e. 144 sub-carriers), which isequivalent to a 2.16 MHz transmission bandwidth. Accordingly, a terminalusing the virtual carrier need only be equipped with a receiver capableof receiving and processing data transmitted over a bandwidth of 2.16MHz. This enables low capability terminals (for example MTC typeterminals) to be provided with simplified receiver units yet still beable to operate within an OFDM type communication network which, asexplained above, conventionally requires terminals to be equipped withreceivers capable of receiving and processing an OFDM signal across theentire bandwidth of the signal.

As explained above, in OFDM-based mobile communication systems such asLTE, downlink data is dynamically assigned to be transmitted ondifferent sub-carriers on a subframe by subframe basis. Accordingly, inevery subframe the network signals which sub-carriers on which symbolscontain data relevant to which terminals (i.e. downlink allocationsignalling).

As can be seen from FIG. 3, in a conventional downlink LTE subframe thisinformation is transmitted on the PDCCH during the first symbol orsymbols of the subframe. However, as previously explained, theinformation transmitted in the PDCCH is spread across the entirebandwidth of the subframe and therefore cannot be received by a mobilecommunication terminal with a simplified receiver unit capable only ofreceiving the reduced bandwidth virtual carrier.

Accordingly, as can be seen in FIG. 5, the final symbols of the virtualcarrier can be reserved as a control region 502 for the virtual carrierfor the transmission of control data indicating which resource elementsof the virtual carrier 501 have been allocated to user equipment (UEs)using the virtual carrier. In some examples the number of symbolscomprising the virtual carrier control region 502 might be fixed, forexample three symbols. In other examples the virtual carrier controlregion 502 can vary in size, for example between one and three symbols,as with the control region 300.

The virtual carrier control region can be located at any suitableposition, for example in the first few symbols of the virtual carrier.In the example of FIG. 5 this could mean positioning the virtual carriercontrol region on the fourth, fifth and sixth symbols. However, fixingthe position of the virtual carrier control region in the final symbolsof the subframe can be useful because the position of the virtualcarrier control region will not vary in dependence on the number ofsymbols of the host carrier control region 300. This can help simplifythe processing undertaken by mobile communication terminals receivingdata on the virtual carrier because there is no need for terminals todetermine a position of the virtual carrier control region everysubframe if it is known that it will always be positioned in the final nsymbols of the subframe.

In a further embodiment, the virtual carrier control symbols mayreference virtual carrier PDSCH transmissions in a separate subframe.

In some examples the virtual carrier may be located within the centreband 310 of the downlink subframe. This can help reduce the impact onhost carrier PDSCH resources caused by the introduction of the virtualcarrier within the host carrier bandwidth since the resources occupiedby the PSS/SSS and PBCH would be contained within the virtual carrierregion and not the remaining host carrier PDSCH region. Therefore,depending on for example the expected virtual carrier throughput, thelocation of a virtual carrier can be appropriately chosen to eitherexist inside or outside the centre band according to whether the host orvirtual carrier is chosen to bear the overhead of the PSS, SSS and PBCH.

Virtual Carrier “Camp-On” Process

As explained above, before a conventional LTE terminal can begintransmitting and receiving data in a cell, it first camps on to thecell. An adapted camp-on process can be provided for terminals using thevirtual carrier.

FIG. 6 shows a flow diagram schematically illustrating a camp-onprocess. There are two branches shown in FIG. 6. Different steps of theprocess associated with a UE intending to use the virtual carrier areshown under the general heading “virtual carrier”. The steps shown underthe general heading “legacy LTE” are associated with a UE intending touse the host carrier, and these steps correspond to the steps of FIG. 4.In this example, the first two steps 400, 401 of the camp-on procedureare common to both the virtual carrier and host (legacy LTE) carrier.

The virtual carrier camp-on process is explained with reference to theexample subframe shown in FIG. 5 in which a virtual carrier with abandwidth of 144 sub-carriers is inserted within the operating bandwidthof a host carrier with a bandwidth corresponding to 1200 sub-carriers.As discussed above, a terminal having a receiver unit with anoperational bandwidth of less than that of the host carrier cannot fullydecode data in the control region of subframes of the host carrier.However, a receiver unit of a terminal having an operational bandwidthof only twelve blocks of twelve sub-carriers (i.e. 2.16 MHz) can receivecontrol and user data transmitted on this example virtual carrier 502.

As noted above, in the example of FIG. 6, the first steps 400 and 401for a virtual carrier terminal are the same as the conventional camp-onprocess shown in FIG. 4, although a virtual carrier terminal may extractadditional information from the MIB as described below. Both types ofterminals (i.e. virtual carrier terminals and host/legacy carrierterminals) can use the PSS/SSS and PBCH to synchronize with the basestation using the information carried on the 72 sub-carrier centre bandwithin the host carrier. However, where the conventional LTE terminalsthen continue with the process by performing the PCFICH decoding step402, which requires a receiver unit capable of receiving and decodingthe host carrier control region 300, a terminal camping on to the cellto receive data on the virtual carrier (which may be referred to as a“virtual carrier terminal”) performs steps 606 and 607 instead.

In a further example a separate synchronisation and PBCH functionalitycan be provided for the virtual carrier device as opposed to re-usingthe same conventional initial camp-on processes of steps 400 and 401 ofthe host carrier device.

At step 606, the virtual carrier terminal locates a virtual carrier, ifany is provided within the host carrier, using a virtualcarrier-specific step. Various examples of how this step may beperformed are discussed further below. Once the virtual carrier terminalhas located a virtual carrier, it can access information within thevirtual carrier. For example, if the virtual carrier mirrors theconventional LTE resource allocation method, the virtual carrierterminal may proceed to decode control portions within the virtualcarrier, which can, for example, indicate which resource elements withinthe virtual carrier have been allocated for a specific virtual carrierterminal or for system information. For example, FIG. 7 shows the blocksof resource elements 350 to 352 within virtual carrier 330 that havebeen allocated for the subframe SF2. However, there is no requirementfor the virtual carrier terminal to follow or mirror the conventionalLTE process (e.g. steps 402-404) and these steps may for example beimplemented very differently for a virtual carrier camp-on process.

Regardless of the virtual carrier terminal following a LTE-like step ora different type of step when performing step 607, the virtual carrierterminal can then decode the allocated resource elements at step 608 andthereby receive data transmitted by the base station broadcasting thevirtual carrier. The data decoded in step 608 may include, for example,the remainder of the system information containing details of thenetwork configuration.

Even though the virtual carrier terminal does not have the bandwidthcapabilities to decode and receive downlink data if it was transmittedin the host carrier using conventional LTE, it can still access avirtual carrier within the host carrier having a limited bandwidthwhilst re-using the initial LTE steps. Step 608 may also be implementedin a LTE-like manner or in a different manner For example, multiplevirtual carrier terminals may share a virtual carrier and have grantsallocated to manage the virtual carrier sharing as shown in SF2 in FIG.7, or, in another example, a virtual carrier terminal may have theentire virtual carrier allocated for its own downlink transmissions, orthe virtual carrier may be entirely allocated to a virtual carrierterminal for a certain number of subframe only, etc.

There is thus a large degree of flexibility provided for the virtualcarrier camp-on process. There is, for example, the ability to adjust abalance between re-using or mirroring conventional LTE steps orprocesses, thereby reducing the terminal complexity and the need toimplement new elements, and adding new virtual carrier specific aspectsor implementations, thereby potentially optimizing the use ofnarrow-band virtual carriers, as LTE has been designed with thelarger-band host carriers in mind

Downlink Virtual Carrier Detection

As discussed above, the virtual carrier terminal should locate (withinthe time-frequency resource grid of the host carrier) the virtualcarrier before it can receive and decode transmissions on the virtualcarrier. Several alternatives are available for the virtual carrierpresence and location determination, which can be implemented separatelyor in combination. Some of these options are discussed below.

To facilitate the virtual carrier detection, the virtual carrierlocation information may be provided to the virtual carrier terminalsuch that it can locate the virtual carrier, if any exists, more easily.For example, such location information may comprise an indication thatone or more virtual carriers are provided within the host carrier, orthat the host carrier does not currently provide any virtual carrier. Itmay also comprise an indication of the virtual carrier's bandwidth, forexample in MHz or blocks of resource elements. Alternatively, or incombination, the virtual carrier location information may comprise thevirtual carrier's centre frequency and bandwidth, thereby giving thevirtual carrier terminal the location and bandwidth of any activevirtual carrier. In the event the virtual carrier is to be found at adifferent frequency position in each subframe, according, for example,to a pseudo-random hopping algorithm, the location information can, forexample, indicate a pseudo random parameter. Such parameters may includea starting frame and parameters used for the pseudo-random algorithm.Using these pseudo-random parameters, the virtual carrier terminal canthen know where the virtual carrier can be found for any subframe.

On implementation feature associated with little change to the virtualcarrier terminal (as compared with a conventional LTE terminal) would beto include location information for the virtual carrier within the PBCH,which already carries the Master Information Block, or MIB in the hostcarrier centre band. As shown in FIG. 8, the MIB consists of 24 bits (3bits to indicate DL bandwidth, 8 bits to indicate the System FrameNumber or SFN, and 3 bits regarding the PHICH configuration). The MIBtherefore comprises 10 spare bits that can be used to carry locationinformation in respect of one or more virtual carriers. For example,FIG. 9 shows an example where the PBCH includes the MIB and locationinformation (“LI”) for pointing any virtual carrier terminal to avirtual carrier.

Alternatively, virtual carrier location information could be provided inthe centre band, outside of the PBCH. It can for example be alwaysprovided after and adjacent to the PBCH. By providing the locationinformation in the centre band but outside of the PBCH, the conventionalPBCH is not modified for the purpose of using virtual carriers, but avirtual carrier terminal can easily find the location information inorder to detect the virtual carrier, if any.

The virtual carrier location information, if provided, can be providedelsewhere in the host carrier, but it may be advantageous to provide itin the centre band, for example because a virtual carrier terminal mayconfigure its receiver to operate on the centre band and the virtualcarrier terminal then does not need to adjust its receiver settings forfinding the location information.

Depending on the amount of virtual carrier location informationprovided, the virtual carrier terminal can either adjust its receiver toreceive the virtual carrier transmissions, or it may require furtherlocation information before it can do so.

If for example, the virtual carrier terminal was provided with locationinformation indicating a virtual carrier presence and/or a virtualcarrier bandwidth but not indicating any details as to the exact virtualcarrier frequency range, or if the virtual carrier terminal was notprovided with any location information, the virtual carrier terminalcould then scan the host carrier for a virtual carrier (e.g. performinga so-called blind search process). Scanning the host carrier for avirtual carrier can be based on different approaches, some of which willbe presented below.

According to a first approach, a virtual carrier might only be insertedin certain pre-determined locations, as illustrated for example in FIG.10 for a four-location example. The virtual carrier terminal then scansthe four locations L1-L4 for any virtual carrier. If and when thevirtual carrier terminal detects a virtual carrier, it can then“camp-on” to the virtual carrier to receive downlink data as describedabove. In this approach, the virtual carrier terminal may be providedwith the possible virtual carrier locations in advance, for example theymay be stored as a network-specific setting in an internal memory.Detection of a virtual carrier could be accomplished by seeking todecode a particular physical channel on the virtual carrier. Thesuccessful decoding of such a channel, indicated for example by asuccessful cyclic redundancy check (CRC) on decoded data, would indicatethe successful location of the virtual carrier.

According to a second approach, the virtual carrier may include locationsignals such that a virtual carrier terminal scanning the host carriercan detect such signals to identify the presence of a virtual carrier.Examples of possible location signals are illustrated in FIGS. 11A to11D. In the examples of FIGS. 11A to 11C, the virtual carrier regularlysends an arbitrary location signal such that a terminal scanning afrequency range where the location signal is would detect this signal.An “arbitrary” signal is intended here to include any signal that doesnot carry any information as such, or is not meant to be interpreted,but merely includes a specific signal or pattern that a virtual carrierterminal can detect. This can for example be a series of positive bitsacross the entire location signal, an alternation of 0 and 1 across thelocation signal, or any other suitable arbitrary signal. It isnoteworthy that the location signal may be made of adjacent blocks ofresource elements or may be formed of non-adjacent blocks. For example,it may be located at every other block of resource elements at the “top”(i.e. upper frequency limit) of the virtual carrier.

In the example of FIG. 11A, the location signal 353 extends across therange R₃₃₀ of the virtual carrier 330 and is always found at the sameposition in the virtual carrier within a subframe. If the virtualcarrier terminal knows where to look for a location signal in a virtualcarrier subframe, it can then simplify its scanning process by onlyscanning this position within a subframe for a location signal. FIG. 11Bshows a similar example where every subframe includes a location signal354 comprising two parts: one at the top corner and one at the bottomcorner of the virtual carrier subframe, at the end of this subframe.Such a location signal may be useful if, for example, the virtualcarrier terminal does not know the bandwidth of the virtual carrier inadvance as it can facilitate a clear detection of the top and bottomfrequency edges of the virtual carrier band.

In the example of FIG. 11C, a location signal 355 is provided in a firstsubframe SF1, but not in a second subframe SF2. The location signal canfor example be provided every two subframes. The frequency of thelocation signals can be chosen to adjust a balance between reducingscanning time and reducing overhead. In other words, the more often thelocation signal is provided, the less long it takes a terminal to detecta virtual carrier but the more overhead there is.

In the example of FIG. 11D, a location signal is provided where thislocation signal is not an arbitrary signal as in FIGS. 11A to 11C, butis a signal that includes information for virtual carrier terminals. Thevirtual carrier terminals can detect this signal when they scan for avirtual carrier and the signal may include information in respect of,for example, the virtual carrier bandwidth or any other virtualcarrier-related information (location or non-location information). Whendetecting this signal, the virtual carrier terminal can thereby detectthe presence and location of the virtual carrier. As shown in FIG. 11D,the location signal can, like an arbitrary location signal, be found atdifferent locations within the subframe, and the location may vary on aper-subframe basis.

Dynamic Variation of Control Region Size of Host Carrier

As explained above, in LTE the number of symbols that make up thecontrol region of a downlink subframe varies dynamically depending onthe quantity of control data that needs to be transmitted. Typically,this variation is between one and three symbols. As will be understoodwith reference to FIG. 5, variation in the width of the host carriercontrol region will cause a corresponding variance in the number ofsymbols available for the virtual carrier. For example, as can be seenin FIG. 5, when the control region is three symbols in length and thereare 14 symbols in the subframe, the virtual carrier is eleven symbolslong. However, if in the next subframe the control region of the hostcarrier were reduced to one symbol, there would be thirteen symbolsavailable for the virtual carrier in that subframe.

When a virtual carrier is inserted into a LTE host carrier, mobilecommunication terminals receiving data on the virtual carrier need to beable to determine the number of symbols in the control region of eachhost carrier subframe to determine the number of symbols in the virtualcarrier in that subframe if they are to be able to use all availablesymbols that are not used by the host carrier control region.

Conventionally, the number of symbols forming the control region issignalled in the first symbol of every subframe in the PCFICH. However,the PCFICH is typically distributed across the entire bandwidth of thedownlink LTE subframe and is therefore transmitted on sub-carriers whichvirtual carrier terminals capable only of receiving the virtual carriercannot receive. Accordingly, in one embodiment, any symbols across whichthe control region could possibly extend are predefined as null symbolson the virtual carrier, i.e. the length of the virtual sub-carrier isset at (m-n) symbols, where m is the total number of symbols in asubframe and n is the maximum number of symbols of the control region.Thus, resource elements are never allocated for downlink datatransmission on the virtual carrier during the first n symbols of anygiven subframe.

Although this embodiment is simple to implement it will be spectrallyinefficient because during subframes when the control region of the hostcarrier has fewer than the maximum number of symbols, there will beunused symbols in the virtual carrier.

In another embodiment, the number of symbols in the control region ofthe host carrier is explicitly signalled in the virtual carrier itself.Once the number of symbols in the control region of the host carrier isknown, the number of symbols in the virtual carrier can be calculated bysubtracting the total number of symbols in the subframe from thisnumber.

In one example an explicit indication of the host carrier control regionsize is given by certain information bits in the virtual carrier controlregion. In other words an explicit signalling message is inserted at apredefined position in the virtual carrier control region 502. Thispredefined position is known by each terminal adapted to receive data onthe virtual carrier.

In another example, the virtual carrier includes a predefined signal,the location of which indicates the number of symbols in the controlregion of the host carriers. For example, a predefined signal could betransmitted on one of three predetermined blocks of resource elements.When a terminal receives the subframe it scans for the predefinedsignal. If the predefined signal is found in the first block of resourceelements this indicates that the control region of the host carriercomprises one symbol; if the predefined signal is found in the secondblock of resource elements this indicates that the control region of thehost carrier comprises two symbols and if the predefined signal is foundin the third block of resource elements this indicates that the controlregion of the host carrier comprises three symbols.

In another example, the virtual carrier terminal is arranged to firstattempt to decode the virtual carrier assuming that the control regionsize of the host carrier is one symbol. If this is not successful, thevirtual carrier terminal attempts to decode the virtual carrier assumingthat the control region size of the host carrier is two and so on, untilthe virtual carrier terminal successfully decodes the virtual carrier.

Downlink Virtual Carrier Reference Signals

As is known in the art, in OFDM-based transmission systems, such as LTE,a number of sub-carriers in symbols throughout the subframes aretypically reserved for the transmission of reference signals. Referencesignals are conventionally transmitted on sub-carriers distributedthroughout a subframe across the channel bandwidth and across the OFDMsymbols. The reference signals are arranged in a repeating pattern andcan be used by a receiver to estimate downlink channel conditions. Thesereference signals are also typically used for additional purposes suchas determining metrics for received signal power indications, automaticfrequency control metrics and automatic gain control metrics. In LTE thepositions of the reference signal bearing sub-carriers within eachsubframe are pre-determined and known at the transceiver of eachterminal.

In a conventional LTE downlink subframes, there are a number ofdifferent reference signals, transmitted for different purposes. Oneexample is the cell-specific reference signal, broadcast to allterminals Cell-specific reference symbols are typically inserted onevery sixth sub-carrier on each transmit antenna port on which theyoccur. Accordingly, if a virtual carrier is inserted in an LTE downlinksubframe, even if the virtual carrier has a minimum bandwidth of oneresource block (i.e. twelve sub-carriers) the virtual carrier willinclude at least some cell-specific reference signal bearingsub-carriers.

There are sufficient reference signal bearing sub-carriers provided ineach subframe such that a receiver need not accurately receive everysingle reference signal to decode the data transmitted in the subframe.However, as will be understood the more reference signals that arereceived, the better a receiver will generally be able to estimate thechannel response, and hence fewer errors will typically be introducedinto the data decoded from the subframe. Accordingly, in order topreserve compatibility with LTE communication terminals receiving dataon the host carrier, in accordance with some example virtual carrierimplementations, the sub-carrier positions that would contain referencesignals in a conventional LTE subframe are retained in the virtualcarrier. These reference symbols can be used by terminal devicesoperating on the virtual carrier for the purpose of measuring channelconditions on the virtual carrier in what is in effect the same way inwhich conventional (legacy) terminal devices can use the referencesymbols for the purpose of measuring channel conditions on the hostcarrier.

As will be understood, terminals arranged to receive only the virtualcarrier receive a reduced number of sub-carriers compared toconventional LTE terminals which receive each subframe across the entirebandwidth of the subframe. As a result, the reduced capability terminalsreceive fewer reference signals over a narrower range of frequencieswhich may result in a less accurate channel estimation being generated.

In some examples a simplified virtual carrier terminal may have a lowermobility which requires fewer reference symbols to support channelestimation. However, in some examples the downlink virtual carrier mayinclude additional reference signal bearing sub-carriers to enhance theaccuracy of the channel estimation (channel condition measurements) thatthe reduced capability terminals can generate (i.e. there may be agreater density of reference symbols on the virtual carrier as comparedto other regions on the host carrier).

In some examples the positions of additional reference bearingsub-carriers are such that they are systematically interspersed withrespect to the positions of the conventional reference signal bearingsub-carriers thereby increasing the sampling frequency of the channelestimation when combined with the reference signals from the existingreference signal bearing sub-carriers. This allows an improved channelestimation of the channel to be generated by the reduced capabilityterminals across the bandwidth of the virtual carrier. In otherexamples, the positions of the additional reference bearing sub-carriersare such that they are systematically placed at the edge of thebandwidth of the virtual carrier thereby increasing the interpolationaccuracy of the virtual carrier channel estimates.

Alternative Virtual Carrier Arrangements

Thus far examples of virtual carrier implementations have primarily beendescribed for the sake of example in terms of a host carrier in which asingle virtual carrier has been inserted, as shown for example in FIG.5. However, as discussed above with reference to FIG. 10, a wirelesstelecommunications system may allow for a plurality of locations for avirtual carrier. Furthermore, referring to the four potential examplelocations schematically represented in FIG. 10, in some cases only oneof the locations may be used for supporting a virtual carrier, whereasin other cases more than one location may be used for simultaneouslysupporting more than one virtual carrier. That is to say, a firstvirtual carrier, VC1, might be supported in the frequency band label L1in FIG. 10, a second virtual carrier, VC2, might be supported in thefrequency band label L2, and respective third and fourth virtualcarriers, VC3 and VC4, might be supported in the frequency bandslabelled L3 and L4. Another example in which a host carrier may supportmore than one virtual carrier is shown for example in FIG. 12. FIG. 12shows an example in which two virtual carriers VC1 (330) and VC2 (331)are simultaneously provided within a host carrier 320. In this example,the two virtual carriers may change location within the host carrierband, for example according to a pseudo-random algorithm. However, inother examples, one or both (or more where more virtual carriers aresupported) of the two virtual carriers may always be found in the samefrequency range within the host carrier frequency range (for example, inline with the locations represented in FIG. 10), or may change positionaccording to a different mechanism. In LTE, the number of virtualcarriers within a host carrier is in principle limited only by the sizeof the host carrier relative to the bandwidths of the virtual carriers.However, in some cases it may be considered that too many virtualcarriers within the host carrier may unduly limit the bandwidthavailable for transmitting data to conventional LTE terminals and anoperator may therefore decide on a number of virtual carrier within ahost carrier according to, for example, a ratio of conventional LTEusers/virtual carrier users.

In some examples the number of active virtual carriers can bedynamically adjusted such that it fits the current needs of conventionalLTE terminals and virtual carrier terminals. For example, if no virtualcarrier terminal is connected or if their access is to be intentionallylimited, the network can arrange to begin scheduling the transmission ofdata to LTE terminals within the sub-carriers previously reserved forthe virtual carrier. This process can be reversed if the number ofactive virtual carrier terminals begins to increase. In some examplesthe number of virtual carriers provided may be increased in response toan increase in the presence of virtual carrier terminals. For example ifthe number of virtual carrier terminals present in a network or area ofa network exceeds a threshold value, an additional virtual carrier isinserted in the host carrier. The network elements and/or networkoperator can thus activate or deactivate the virtual carriers wheneverappropriate.

The virtual carrier shown for example in FIG. 5 is 144 sub-carriers inbandwidth. However, in other examples a virtual carrier may be of anysize between twelve sub-carriers to 1188 sub-carriers (for a carrierwith a 1200 sub-carrier transmission bandwidth). Because in LTE thecentre band has a bandwidth of 72 sub-carriers, a virtual carrierterminal in an LTE environment preferentially has a receiver bandwidthof at least 72 sub-carriers (1.08 MHz) such that it can decode thecentre band 310, therefore a 72 sub-carrier virtual carrier may providea convenient implementation option. With a virtual carrier comprising 72sub-carriers, the virtual carrier terminal does not have to adjust thereceiver's bandwidth for camping on the virtual carrier which maytherefore reduce complexity of performing the camp-on process, but thereis no requirement to have the same bandwidth for the virtual carrier asfor the centre band and, as explained above, a virtual carrier based onLTE can be of any size between 12 to 1188 sub-carriers. For example, insome systems, a virtual carrier having a bandwidth of less than 72sub-carriers may be considered as a waste of the virtual carrierterminal's receiver resources, but from another point of view, it may beconsidered as reducing the impact of the virtual carrier on the hostcarrier by increasing the bandwidth available to conventional LTEterminals The bandwidth of a virtual carrier can therefore be adjustedto achieve the desired balance between complexity, resource utilization,host carrier performance and requirements for virtual carrier terminals.

Uplink Transmission Frame

So far, the virtual carrier has been discussed primarily with referenceto the downlink, however in some examples a virtual carrier can also beinserted in the uplink.

In frequency division duplex (FDD) networks both the uplink and downlinkare active in all subframes, whereas in time division duplex (TDD)networks subframes can either be assigned to the uplink, to thedownlink, or further sub-divided into uplink and downlink portions.

In order to initiate a connection to a network, conventional LTEterminals make a random access request on the physical random accesschannel (PRACH). The PRACH is located in predetermined blocks ofresource elements in the uplink frame, the positions of which aresignaled to the LTE terminals in the system information signaled on thedownlink

Additionally, when there is pending uplink data to be transmitted froman LTE terminal and the terminal does not already have any uplinkresources allocated to it, it can transmit a random access request PRACHto the base station. A decision is then made at the base station as towhich if any uplink resource is to be allocated to the terminal devicethat has made the request. Uplink resource allocations are then signaledto the LTE terminal on the physical downlink control channel (PDCCH)transmitted in the control region of the downlink subframe.

In LTE, transmissions from each terminal device are constrained tooccupy a set of contiguous resource blocks in a frame. For the physicaluplink shared channel (PUSCH) the uplink resource allocation grantreceived from the base station will indicate which set of resourceblocks to use for that transmission, where these resource blocks couldbe located anywhere within the channel bandwidth.

The first resources used by the LTE physical uplink control channel(PUCCH) are located at both the upper and lower edge of the channel,where each PUCCH transmission occupies one resource block. In the firsthalf of a subframe this resource block is located at one channel edge,and in the second half of a subframe this resource block is located atthe opposite channel edge. As more PUCCH resources are required,additional resource blocks are assigned in a sequential manner, movinginward from the channel edges. Since PUCCH signals are code divisionmultiplexed, an LTE uplink can accommodate multiple PUCCH transmissionsin the same resource block.

Virtual Uplink Carrier

In accordance with some implementations, the virtual carrier terminalsdescribed above can also be provided with a reduced capabilitytransmitter for transmitting uplink data. The virtual carrier terminalsare arranged to transmit data across a reduced bandwidth. The provisionof a reduced capability transmitter unit provides correspondingadvantages to those achieved by providing a reduced capability receiverunit with, for example, classes of devices that are manufactured with areduced capability for use with, for example, MTC type applications.

In correspondence with the downlink virtual carrier, the virtual carrierterminals transmit uplink data across a reduced range of sub-carrierswithin a host carrier that has a greater bandwidth than that of thereduced bandwidth virtual carrier. This is shown in FIG. 13A. As can beseen from FIG. 13A, a group of sub-carriers in an uplink subframe form avirtual carrier 1301 within a host carrier 1302. Accordingly, thereduced bandwidth across which the virtual carrier terminals transmituplink data can be considered a virtual uplink carrier.

In order to implement the virtual uplink carrier, the base stationscheduler serving a virtual carrier ensures that all uplink resourceelements granted to virtual carrier terminals are sub-carriers that fallwithin the reduced bandwidth range of the reduced capability transmitterunits of the virtual carrier terminals Correspondingly, the base stationscheduler serving the host carrier typically ensures that all uplinkresource elements granted to host carrier terminals are sub-carriersthat fall outside the set of sub-carriers occupied by the virtualcarrier terminals. However, if the schedulers for the virtual carrierand the host carrier are implemented jointly, or have means to shareinformation, then the scheduler of the host carrier can assign resourceelements from within the virtual carrier region to terminal devices onthe host carrier during subframes when the virtual carrier schedulerindicates that some or all of the virtual carrier resources will not beused by terminal devices on the virtual carrier.

If a virtual carrier uplink incorporates a physical channel that followsa similar structure and method of operation to the LTE PUCCH, whereresources for that physical channel are expected to be at the channeledges, for virtual carrier terminals these resources could be providedat the edges of the virtual carrier bandwidth and not at the edges ofthe host carrier. This is advantageous since it would ensure thatvirtual carrier uplink transmissions remain within the reduced virtualcarrier bandwidth.

Virtual Uplink Carrier Random Access

In accordance with conventional LTE techniques, it cannot be guaranteedthat the PRACH will be within the sub-carriers allocated to the virtualcarrier. In some embodiments therefore, the base station provides asecondary PRACH within the virtual uplink carrier, the location of whichcan be signaled to the virtual carrier terminals via system informationon the virtual carrier. This is shown for example in FIG. 13B in which aPRACH 1303 is located within the virtual carrier 1301. Thus, the virtualcarrier terminals send PRACH requests on the virtual carrier PRACHwithin the virtual uplink carrier. The position of the PRACH can besignaled to the virtual carrier terminals in a virtual carrier downlinksignaling channel, for example in system information on the virtualcarrier.

However, in other examples, the virtual carrier PRACH 1303 is situatedoutside of the virtual carrier as shown for example in FIG. 13C. Thisleaves more room within the virtual uplink carrier for the transmissionof data by the virtual carrier terminals. The position of the virtualcarrier PRACH is signaled to the virtual carrier terminals as before butin order to transmit a random access request, the virtual carrierterminals re-tune their transmitter units to the virtual carrier PRACHfrequency because it is outside of the virtual carrier. The transmitterunits are then re-tuned to the virtual carrier frequency when uplinkresource elements have been allocated.

In some examples where the virtual carrier terminals are capable oftransmitting on a PRACH outside of the virtual carrier, the position ofthe host carrier PRACH can be signaled to the virtual carrier terminalsThe virtual carrier terminals can then simply use the conventional hostcarrier PRACH resource to send random access requests. This approach isadvantageous as fewer PRACH resources have to be allocated.

However, if the base station is receiving random access requests fromboth conventional LTE terminals and virtual carrier terminals on thesame PRACH resource, it is necessary that the base station is providedwith a mechanism for distinguishing between random access requests fromconventional LTE terminals and random access requests from virtualcarrier terminals.

Therefore, in some examples a time division allocation is implemented atthe base station whereby, for example, over a first set of subframes thePRACH allocation is available to the virtual carrier terminals and overa second set of subframes the PRACH allocation is available toconventional LTE terminals. Accordingly, the base station can determinethat random access requests received during the first set of subframesoriginate from virtual carrier terminals and random access requestsreceived during the second set of subframes originate from conventionalLTE terminals.

In other examples, no mechanism is provided to prevent both virtualcarrier terminals and conventional LTE terminals transmitting randomaccess requests at the same time. However, the random access preamblesthat are conventionally used to transmit a random access request aredivided into two groups. The first group is used exclusively by virtualcarrier terminals and the second group is used exclusively byconventional LTE terminals. Accordingly, the base station can determinewhether a random request originated from a conventional LTE terminal ora virtual carrier terminal simply by ascertaining to what group therandom access preamble belongs.

Example Architecture

FIG. 14 provides a schematic diagram showing part of an adapted LTEmobile telecommunication system arranged in accordance with an exampleof the present invention. The system includes an adapted enhanced Node B(eNB/base station) 1401 connected to a core network 1408 whichcommunicates data to a plurality of conventional LTE terminals 1402 andreduced capability terminals 1403 within a coverage area (cell) 1404.Each of the reduced capability terminals 1403 has a tuneable transceiverunit 1405 which includes a receiver unit capable of receiving dataacross a reduced bandwidth and a transmitter unit capable oftransmitting data across a reduced bandwidth when compared with thecapabilities of the transceiver units 1406 included in the conventionalLTE terminals 1402.

The adapted eNB 1401 is arranged to transmit downlink data using asubframe structure that includes a virtual carrier as described above,for example with reference to FIG. 5, and to receive uplink data using asubframe structure as described with reference to FIG. 13B or 13C. Thereduced capability terminals 1403 are thus able to receive and transmitdata using the uplink and downlink virtual carriers as described above.

As has been explained above, because the reduced complexity terminals1403 receive and transmit data across a reduced bandwidth on the uplinkand downlink virtual carriers, the complexity, power consumption andcost of the transceiver unit 1405 needed to receive and decode downlinkdata and to encode and transmit uplink data is reduced compared to thetransceiver unit 1406 provided in the conventional LTE terminals.

When receiving downlink data from the core network 1408 to betransmitted to one of the terminals within the cell 1404, the adaptedeNB 1401 is arranged to determine if the data is bound for aconventional LTE terminal 1402 or a reduced capability terminal 1403.This can be achieved using any suitable technique. For example, databound for a reduced capability terminal 1403 may include a virtualcarrier flag indicating that the data must be transmitted on thedownlink virtual carrier. If the adapted eNB 1401 detects that downlinkdata is to be transmitted to a reduced capability terminal 1403, anadapted scheduling unit 1409 included in the adapted eNB 1401 ensuresthat the downlink data is transmitted to the reduced capability terminalin question on the downlink virtual. In another example the network isarranged so that the virtual carrier is logically independent of theeNB. More particularly the virtual carrier may be arranged to appear tothe core network as a distinct cell so that it is not known to the corenetwork that the virtual carrier has any relationship with the hostcarrier. Packets are simply routed to/from the virtual carrier just asthey would be for a conventional cell.

In another example, packet inspection is performed at a suitable pointwithin the network to route traffic to or from the appropriate carrier(i.e. the host carrier or the virtual carrier).

In yet another example, data from the core network to the eNB iscommunicated on a specific logical connection for a specific terminaldevice. The eNB is provided with information indicating which logicalconnection is associated with which terminal device. Information is alsoprovided at the eNB indicating which terminal devices are virtualcarrier terminals and which are conventional LTE terminals. Thisinformation could be derived from the fact that a virtual carrierterminal would initially have connected using virtual carrier resources.In other examples virtual carrier terminals are arranged to indicatetheir capability to the eNB during the connection procedure. Accordinglythe eNB can map data from the core network to a specific terminal devicebased on whether the terminal device is a virtual carrier terminal or anLTE terminal

When scheduling resources for the transmission of uplink data, theadapted eNB 1401 is arranged to determine if the terminal to bescheduled resources is a reduced capability terminal 1403 or aconventional LTE terminal 1402. In some examples this is achieved byanalysing the random access request transmitted on the PRACH using thetechniques to distinguish between a virtual carrier random accessrequest and a conventional random access request as described above. Inany case, when it has been determined at the adapted eNB 1401 that arandom access request has been made by a reduced capability terminal1402, the adapted scheduler 1409 is arranged to ensure that any grantsof uplink resource elements are within the virtual uplink carrier.

In some examples, the virtual carrier inserted within the host carriercan be used to provide a logically distinct “network within a network”.In other words data being transmitted via the virtual carrier can betreated as logically and physically distinct from the data transmittedby the host carrier network. The virtual carrier can therefore be usedto implement what might be called a dedicated messaging network (DMN)which is “laid over” a conventional network and used to communicatemessaging data to DMN devices (i.e. virtual carrier terminals), forexample classes of MTC devices.

Further Example Applications of Virtual Carriers

Having set out the concepts of virtual carriers of the kind described inco-pending UK patent applications numbered GB 1101970.0 [2], GB1101981.7 [3], GB 1101966.8 [4], GB 1101983.3 [5], GB 1101853.8 [6], GB1101982.5 [7], GB 1101980.9 [8] and GB 1101972.6 [9], GB 1121767.6 [10]and GB 1121766.8 [11], some other extensions of the virtual carrierconcept in accordance with some embodiments of the invention aredescribed.

FIG. 15 schematically shows a telecommunications system 1500 accordingto an embodiment of the invention. The telecommunications system 1500 inthis example is based broadly on an LTE-type architecture in whichvirtual carriers, such as described above, are implemented. As such manyaspects of the operation of the telecommunications system 1500 are knownand understood and are not described here in detail in the interest ofbrevity. Operational aspects of the telecommunications system 1500 whichare not specifically described herein may be implemented in accordancewith any known techniques, for example according to the currentLTE-standards with appropriate modifications to support virtual carriersas has been previously proposed.

The telecommunications system 1500 comprises a core network part(evolved packet core) 1502 coupled to a radio network part. The radionetwork part comprises a base station (evolved-nodeB) 1504 coupled to aplurality of terminal devices. In this example, two terminal devices areshown, namely a first terminal device 1506 and a second terminal device1508. It will of course be appreciated that in practice the radionetwork part may comprise a plurality of base stations serving a largernumber of terminal devices across various communication cells. However,only a single base station and two terminal devices are shown in FIG. 15in the interests of simplicity.

As with a conventional mobile radio network, the terminal devices 1506,1508 are arranged to communicate data to and from the base station(transceiver station) 1504. The base station is in turn communicativelyconnected to a serving gateway, S-GW, (not shown) in the core networkpart which is arranged to perform routing and management of mobilecommunications services to the terminal devices in thetelecommunications system 1500 via the base station 1504. In order tomaintain mobility management and connectivity, the core network part1502 also includes a mobility management entity (not shown) whichmanages the enhanced packet service, EPS, connections with the terminaldevices 1506, 1508 operating in the communications system based onsubscriber information stored in a home subscriber server, HSS. Othernetwork components in the core network (also not shown for simplicity)include a policy charging and resource function, PCRF, and a packet datanetwork gateway, PDN-GW, which provides a connection from the corenetwork part 1502 to an external packet data network, for example theInternet. As noted above, the operation of the various elements of thecommunications system 1500 shown in FIG. 15 may be broadly conventionalapart from where modified to provide functionality in accordance withembodiments of the invention as discussed herein.

In this example, it is assumed the first terminal device 1506 is aconventional smart-phone type terminal device communicating with thebase station 1504 primarily using resources associated with the hostcarrier component of the radio interface. This first terminal device1504 comprises a transceiver unit 1506 a for transmission and receptionof wireless signals and a controller unit 1506 b configured to controlthe smart phone 1506. The controller unit 1506 b may comprise aprocessor unit which is suitably configured/programmed to provide thedesired functionality using conventional programming/configurationtechniques for equipment in wireless telecommunications systems. Thetransceiver unit 1506 a and the controller unit 1506 b are schematicallyshown in FIG. 15 as separate elements. However, it will be appreciatedthat the functionality of these units can be provided in variousdifferent ways, for example using a single suitably programmedintegrated circuit. As will be appreciated the smart phone 1506 will ingeneral comprise various other elements associated with its operatingfunctionality.

In this example, it is assumed the second terminal device 1508 is amachine-type communication (MTC) terminal device communicating with thebase station 1504 using resources associated with a virtual carriercomponent of the radio interface. Furthermore, it is assumed for thisexample the base station 1504 is able to support multiple virtualcarriers at different frequency locations within the host carrier, forexample as schematically represented in FIGS. 10 and 12, and that thesecond terminal device 1508 is frequency tunable such that it is able toselectively operate on different ones of the virtual carrierfrequencies. As discussed above, machine-type communication terminaldevices can in some cases be typically characterised as semi-autonomousor autonomous wireless communication devices communicating small amountsof data. Examples include so-called smart meters which, for example, maybe located in a customer's house and periodically transmit informationback to a central MTC server data relating to the customer's consumptionof a utility such as gas, water, electricity and so on. MTC devices mayin some respects be seen as devices which can be supported by relativelylow bandwidth communication channels having relatively low quality ofservice (QoS), for example in terms of latency. It is assumed here theMTC terminal device 1508 in FIG. 15 is such a device.

As with the smart phone 1506, the MTC device 1508 comprises atransceiver unit 1508 a for transmission and reception of wirelesssignals and a controller unit 1508 b configured to control the MTCdevice 1508. The transceiver unit 1508 a is tunable such that it may betuned to receive communications from the base stations within itsoperating bandwidth at different frequency locations within the overalloperating bandwidth of the base station (corresponding here to thebandwidth of the host carrier). The controller unit 1508 b may comprisevarious sub-units for providing functionality in accordance withembodiments of the invention as explained further below. These sub-unitsmay be implemented as discrete hardware elements or as appropriatelyconfigured functions of the controller unit. Thus the controller unit1508 b may comprise a processor unit which is suitablyconfigured/programmed to provide the desired functionality describedherein using conventional programming/configuration techniques forequipment in wireless telecommunications systems. The transceiver unit1508 a and the controller unit 1508 b are schematically shown in FIG. 15as separate elements for ease of representation. However, it will beappreciated that the functionality of these units can be provided invarious different ways following established practices in the art, forexample using a single suitably programmed integrated circuit. It willbe appreciated the MTC device 1508 will in general comprise variousother elements associated with its operating functionality.

The base station 1504 comprises a transceiver unit 1504 a fortransmission and reception of wireless signals and a controller unit1504 b configured to control the base station 1504. As noted above, thetransceiver unit 1504 a of the base station 1504 is configured tosupport multiple virtual carriers within a host carrier. The controllerunit 1506 b may again comprise various sub-units, such as a schedulingunit, for providing functionality in accordance with embodiments of theinvention as explained further below. These sub-units may be implementedas discrete hardware elements or as appropriately configured functionsof the controller unit. Thus, the controller unit 1504 b may comprise aprocessor unit which is suitably configured/programmed to provide thedesired functionality described herein using conventionalprogramming/configuration techniques for equipment in wirelesstelecommunications systems. The transceiver unit 1504a and thecontroller unit 1504b are schematically shown in FIG. 15 as separateelements for ease of representation. However, it will be appreciatedthat the functionality of these units can be provided in variousdifferent ways following established practices in the art, for exampleusing a single suitably programmed integrated circuit. It will beappreciated the base station 1504 will in general comprise various otherelements associated with its operating functionality.

Thus, the base station 1504 is configured to communicate data with thesmart phone 1506 over a first radio communication link 1510 associatedwith a host carrier of the wireless telecommunications system and tocommunicate data with the MTC device 1508 over a second radiocommunication link 1512 associated with a virtual carrier of thewireless application system, and wherein the virtual carrier supportingthe second radio communication link is one of a plurality of virtualcarriers supported by the base station 1504. It is assumed here the basestation 1504 is configured to communicate with the smart phone 1506 overthe first radio communication link 1510 in accordance with theestablished principles of LTE-based communications supporting a host andvirtual carrier, such as described above.

A significant aspect of the wireless telecommunications system 1500schematically represented in FIG. 15 is that the base station isconfigured to communicate data on virtual carriers at differentlocations within the base station's overall operating bandwidth (i.e.the base station supports multiple virtual carriers at differentfrequency locations). For the sake of a concrete example, it is assumedhere the base station is associated with an overall operating bandwidthof the 20 MHz, and supports virtual carrier communications with reducedcapability devices, such as MTC-type terminal device 1508, using fourvirtual carriers with each one having a restricted frequency bandwidthof 1.4 MHz. Furthermore, it is assumed in this example the four virtualcarriers are at fixed frequency locations evenly distributed across theoverall operating bandwidth of 20 MHz. Thus in this example the basestation supports a first virtual carrier, VC 1, centred around afrequency location of 2.5 MHz relative to lower edge of its overall 20MHz bandwidth, a second virtual carrier, VC 2, centred around afrequency location of 7.5 MHz from the lower edge of the host bandwidth,a third virtual carrier, VC 3, centred around a frequency location of12.5 MHz from the lower edge of the host bandwidth, and a fourth virtualcarrier, VC 4, centred around a frequency location of 17.5 MHz from thelower edge of the host bandwidth. It will be appreciated these valuessimply represent one example implementation, and in other examples theremay be different numbers of virtual carriers and/or the virtual carriersmay be distributed differently throughout the host carrier bandwidth. Inthe example represented in FIG. 15 it is assumed the base station 1504is initially communicating with the MTC device 1508 using frequencyresources associated with the first virtual carrier VC 1. Thus, thetunable transceiver 1508 a of the MTC terminal device 1508 isappropriately tuned to the frequency location associated with virtualcarrier VC 1. The MTC terminal device 1508 may currently be operating onthe virtual carrier VC 1 simply because this was the first virtualcarrier the MTC device found during a camp-on procedure or because theMTC device is configured to always initially camp-on to a virtualcarrier VC 1, or because the base station has previously instructed theMTC device to operate on this particular virtual carrier. For example,the base station may be configured to initially allocate different MTCterminal devices to different virtual carriers to manage/balance loadingon the different virtual carriers and to provide appropriate instructionsignalling to the respective MTC devices as regards which virtualcarrier they should be using, for example with signalling exchangedduring an initial attach procedure.

Thus, FIG. 15 represents a situation in which the MTC device 1508 isoperating within the wireless telecommunications system 1500 bycommunicating with the base station on a restricted bandwidth frequencycarrier at the frequency location associated with the first virtualcarrier VC1 supported by the base station. The operation of the MTCdevice on this virtual carrier may largely follow thepreviously-proposed ideas for operating reduced capability terminaldevices on virtual carriers, but with modifications in accordance withembodiments of the invention as discussed further below.

As has been previously proposed for virtual carrier implementations,communications between the base station 1504 and the MTC device 1508 mayin many respects follow broadly conventional LTE-type techniques (orcorresponding techniques in the context of wireless telecommunicationssystems operating according to other standards). For example, referencesignalling within the bandwidth of the virtual carrier may be used bythe MTC device to measure channel conditions existing between the basestation and the MTC device, and the MTC device may report an indicationof the measurements to the base station (e.g. a CQI type report for thevirtual carrier). Thus the base station may be provided with informationon channel conditions for the virtual carrier to allow the base stationto perform link adaptation in respect of communications with the MTCdevice on the virtual carrier in broadly the same manner as forconventional LTE-type communications, albeit within a restrictedbandwidth.

The inventors have recognised an aspect of virtual carrier operationwhich does not apply for conventional LTE modes of operation is thepotential for in effect switching an MTC device from being served on onevirtual carrier to being served on another virtual carrier. For example,with reference to the specific example described above in which MTCdevice 1508 is being served on virtual carrier VC 1, there is inprinciple the potential for communications with the MTC device 1508 toswitch to another one of the four virtual carriers supported in thisparticular example. The inventors have recognised that this allows forwhat is in effect another level of potential link adaptation that may beapplied to optimise communications between the base station and the MTCdevice. In order to take advantage of this possibility, the inventorshave developed approaches which allow an MTC device operating on arestricted frequency bandwidth within a wider system bandwidth tomeasure channel conditions at different locations within the systembandwidth and to provide information derived from such measurements to asupporting base station. This can therefore allow the base station totake account of the measured channel conditions at different frequencylocations within the system bandwidth when scheduling frequencylocations for supporting the MTC device. For example, the base stationmay select a particular virtual carrier frequency to use forcommunications with the MTC device based on the measured channelconditions.

FIG. 16 is a signalling ladder diagram schematically representingcommunications between a base station and a terminal device and stepsperformed by the base station and terminal device in accordance withcertain embodiments of the invention. In this example it is assumed thebase station and the terminal device correspond with the base station1504 and the terminal device 1508 schematically represented in FIG. 15with the terminal device 1508 initially attached to the base stationusing virtual carrier VC 1.

The signalling represented in FIG. 16 starts from a point at which thebase station 1504 has determined that the terminal device should measurechannel conditions in accordance with an embodiment of the invention.This might be, for example, because the base station is configured torequest such measurements according to a predefined schedule, or perhapsbecause the base station has recognised that communications with theterminal device are being impacted by poor channel conditions on thecurrently used virtual carrier (for example based on conventionalLTE-type channel condition reporting within the bandwidth of the virtualcarrier).

Thus in a first step S1 represented in FIG. 16 the base station 1504transmits signalling to the MTC device 1508 to instruct the terminaldevice to perform channel condition measurements in accordance with anembodiment of the invention. As noted above, it is assumed the terminaldevice is initially operating on a virtual carrier VC 1 and so thissignalling is correspondingly sent by the base station to the MTC deviceon VC1. The instruction signalling S1 may comprise configurationinformation to assist the terminal device. For example, the instructionsignalling sent in step Si may comprise an indication of the frequencylocations for which the base station would like the terminal device tomeasure channel conditions. These may be provided, for example, as anindication of specific subcarrier frequencies for which measurements aredesired, or by reference to one or more predefined frequency locations,for example by way of an index referring to certain pre-defined virtualcarrier frequencies supported by the base station. For example, in theabove-described case where the base station supports four virtualcarriers which might be referred to as VC 1, VC 2, VC 3 and VC 4, theconfiguration messaging may comprise an indication of one or more of therespective indices 1 to 4 for which the base station would like theterminal device to perform channel condition measurements. Otherinformation that may be conveyed by the base station regarding themeasures to be made might include, for example, an indication of thebandwidth over which the measurements are to be made (e.g. where this isnot fixed). In other examples there may be no additional configurationinformation beyond an indication that the base station should initiatechannel condition measurement and reporting in accordance with anembodiment of the invention. In such cases the terminal device might,for example, be configured to perform the measurements in a predefinedmanner (for example, by taking measurements for all virtual carrierfrequencies supported by the base station in the implementation athand). For the example schematically represented in FIG. 16 it will beassumed the base station instructs the terminal device to make channelcondition measurements for all four virtual carriers which the basestation supports in this example implementation. The signalling sent instep S1 of FIG. 16 may be transmitted in accordance with anyconventional technique for conveying control information from a basestation to a terminal device in a wireless telecommunications system.

In step S2 represented in FIG. 16, the base station conveys an uplinkresource allocation to the terminal device which the terminal deviceshould later use to report information derived from the measurements ithas been instructed to make It will be appreciated the timing for theallocated uplink resources will be delayed to allow the terminal devicetime to perform the instructed measurements. Because of this it may beappropriate for the uplink resource allocation to be made in a differentway from conventional uplink resource allocations, for example any ofthe established techniques for higher layer control signalling may beused to convey an indication of the resources on which the terminaldevice is to report on the measurements. As with the signallingrepresented in step S1, this signalling represented in step S2 may besent on virtual carrier VC 1 using established techniques for conveyingcontrol information in a wireless telecommunications networkimplementing a virtual carrier.

Having received the instruction to perform channel conditionmeasurements and having determined the frequency locations for whichchannel condition measurements are to be made (either based onconfiguration information received from the base station, or inaccordance with a pre-defined fixed scheme) the terminal device proceedsto make these measurements as schematically represented in steps S3 toS9 of FIG. 16. Thus in step S3 the terminal device measures the channelconditions for VC 1. This is the virtual carrier to which the terminaldevice is initially tuned. The channel conditions for VC 1 may bemeasured in accordance with any conventional techniques. For example,relying on reference signalling measurements to established a CQIparameter for VC 1. In this example it is assumed only a single CQI(i.e. corresponding to a wideband CQI) is established for the virtualcarrier bandwidth VC 1. Having made the measurement, the terminal devicestores an indication of the results in a memory and proceeds to step S4represented in FIG. 16.

In step S4 the terminal device tunes its transceiver to correspond witha frequency location for the second virtual carrier VC 2. When theterminal device transceiver has been retuned to a frequency locationcorresponding to VC 2 the terminal device proceeds to measure channelconditions for VC 2 in step S5 and record the results in memory. Thismeasurement may again be made in accordance with generally conventionaltechniques for measuring channel conditions in wirelesstelecommunications systems. For example, the measurement might comprisea measurement of a received reference signal and a measurement of noise(measurement of noise plus interference) in the usual way. In the sameway the terminal device proceeds to retune itself to a frequencylocation corresponding to virtual carrier VC 3 in step S6, and measurechannel conditions for VC 3 in step S7 and to retune itself to afrequency location corresponding to virtual carrier VC 4 in step S8, andmeasure channel conditions for VC 4 in step S9.

Having completed the channel condition measurements for the frequencylocations of interest, the terminal device retunes its transceiver backto a frequency location corresponding to virtual carrier VC 1, asschematically represented in step S10 of FIG. 16.

Thus, after step S10 represented in FIG. 16 the terminal device hassequentially measured channel conditions for each virtual carrier VC 1,VC 2, VC 3 and VC 4, stored a corresponding indication of the results ofthe measurements in a memory, and retuned its transceiver back tovirtual carrier VC 1. The terminal device in accordance with embodimentsof the invention is configured to derive information from themeasurements and communicate this information to the base station in achannel condition measurement report, as schematically represented instep S11 of FIG. 16. This communication is sent on the resourcesallocated to the terminal device for uplink communication in step S2.

The information communicated from the terminal device to the basestation may be different in accordance with different exampleimplementations. For the specific example represented in FIG. 16 theterminal device is configured to determine from the respective channelcondition measurements which virtual carrier frequency location isassociated with the best channel conditions and to communicate this tothe base station along with an indication of the correspondingmeasurement of channel conditions. That is to say, the informationconveyed in the signalling represented in step S11 corresponds with anindication of a frequency location for a virtual carrier which theterminal device has selected as being associated with the best channelconditions from those which have been measured and an indication of whatthose best channel conditions are. Additional information, such as anindication of the time at which the relevant measurements were made, mayalso be communicated to the base station. The indication of thefrequency location determined by the terminal device as being associatedwith the best channel conditions may thus be considered as being anindication of a terminal device selected frequency suggestion to use forsubsequent virtual carrier operation.

On receiving the information from the terminal device, the base stationmakes scheduling decisions for the terminal device in a manner whichtakes account of the information, as schematically represented in stepS12. For example, if the terminal device is initially operating on VC 1,but the information communicated in step S11 of FIG. 16 indicates theterminal device has determined from its measurements that virtualcarrier VC 3 provides better channel conditions, the base station maydetermine that future scheduling for the terminal device should be movedto virtual carrier VC 3. If the base station determines from theinformation received from the terminal device that the terminal devicewould be better served on a different virtual carrier, and furthermorethe base station is able to support the terminal device on that virtualcarrier (i.e. there is capacity for the mobile terminal on the suggestedvirtual carrier), the base station may provide instruction signalling tothe terminal device to instruct the terminal device to move to theselected virtual carrier. This is schematically represented in FIG. 16by signalling step S13. This signalling may be made in accordance withany established techniques for conveying control signalling in wirelesstelecommunications systems.

On receiving such an instruction the terminal device may retune itstransceiver and attach to the selected virtual carrier in accordancewith conventional techniques, and thereby begin operating on thenewly-selected virtual carrier (not shown in FIG. 16).

Thus, the approach of FIG. 16 represents a technique whereby a terminaldevice with a reduced bandwidth capability can provide feedback to abase station regarding channel conditions across a wider range offrequencies than the terminal device's operating bandwidth, therebyallowing the base station to determine whether the terminal device maybe better served (for example in terms of achieving higher data rates)if it were to be moved to operate on a different virtual carrierfrequency.

It will of course be appreciated the operation represented in FIG. 16might be modified in various ways in accordance with otherimplementations.

For example, instead of the terminal device communicating to the basestation an indication of which virtual carrier frequency location ismeasured as being associated with the best channel conditions, theterminal device might communicate different and/or additionalinformation derived from the measurements to the base station in a stepcorresponding to step S11 in FIG. 16.

In some embodiments the terminal device having made the measurements ofchannel conditions associated with a plurality of different potentialvirtual carrier locations might communicate an indication of therespective channel condition measurements to the base station. That isto say, the information derived from the channel condition measurementsand communicated to the base station may comprise an indication of therespective measurements (or a subset of the respective measurements),for example parameterised in terms of a CQI-type parameter for eachmeasurement. This provides the base station with information that wouldallow the base station to itself select the most appropriate virtualcarrier for the terminal device. This approach provides the base stationresponsible for overall scheduling in the cell with more informationwhich can help optimise the overall performance within the cellsupported by the base station. For example, if the base station cannotaccommodate the terminal device on the virtual carrier associated withthe best channel conditions, for example because of overcapacity, thebase station can instead consider moving the terminal device to thevirtual carrier having the second best channel conditions. This approachwould not be readily available if the terminal device were to reportonly the single virtual carrier with the best channel conditions.However, an advantage of the terminal device reporting only on thesingle virtual carrier with the best channel conditions is a reductionin the amount of data to be exchanged and a reduction in the processingrequirements of the base station (because the terminal device processesthe channel condition measurements to determine the best virtual carrierto use). An intermediate approach would be for the terminal device toreport information regarding a subset of the virtual carriers'respective channel condition measurements. For example, instead ofcommunicating information relating to the virtual carrier frequencylocation determined to have the best channel conditions, the terminaldevice might communicate an indication of which virtual carrierfrequency locations are determined to have the best and second bestchannel conditions (and perhaps third best, and fourth best, and so on).In another example, the terminal device might communicate an indicationof which virtual carrier frequency locations are associated withmeasured channel conditions meeting some predefined criterion, forexample for which frequency locations a parameter derived from thechannel condition measurements exceeds a pre-defined performancethreshold value.

Furthermore, it will be appreciated other example implementations mightnot comprise all of the steps represented in FIG. 16 and/or mightinclude corresponding steps performed in a different way. For example,in some embodiments the information conveyed in steps S1 and S2 might beconveyed from the base station to the terminal device in a singlemessaging step. In other examples an uplink resource allocation such asrepresented in step S2 might not be provided. Instead, the terminaldevice 1508 might simply perform the measurement under instructionreceived in step S1, and once the measurement is completed (i.e. afterstep S10 in FIG. 16) the terminal device might at that stage requestuplink resources to allow it to send the information derived from themeasurements to the base station in step S11. For example, having madethe channel condition measurements and derived the information to becommunicated to the base station, the terminal device might at thatstage request resources through a conventional scheduling requestprocedure, for example using a RACH procedure. Alternatively, thesignalling represented in step S2 might be provided at a later stage.For example, the base station may be configured to send an uplinkresource allocation for the terminal device to use to report informationderived from the measurements after the measurements have been made. Forexample, the base station might send signalling corresponding to thatrepresented in step S1 of FIG. 16 to initiate the channel conditionmeasurements, and then, after a given period of time has passed to allowthe terminal device to make the measurements, the base station mightsend signalling corresponding to that represented in step S2.

In yet other examples, there may be no signalling corresponding to stepsS1 and S2. Rather, the terminal device 1508 might, for example, beconfigured to initiate the measurement steps corresponding to thosestarting from step S3 of the operation represented in FIG. 16 withoutinstruction from the base station. For example, the terminal devicemight be configured to do this according to a pre-defined schedule, orbecause the terminal device itself identifies that channel conditionshave deteriorated on a currently used virtual carrier. When the terminaldevice has completed its measurements, it may request uplink resourcesin the usual way to allow it to convey the fact it has taken themeasurements and the information derived from the measurements, to thebase station. In other examples signalling such as represented in stepS1 and/or step S2 FIG. 16 may be conveyed implicitly, rather thanexplicitly. For example, rather than send signalling corresponding tostep S2 in FIG. 16, a wireless telecommunications system might beconfigured such that on receipt of signalling corresponding to step S1,the terminal device 1508 derives an uplink resource to be used forreporting the information derived from the measurements based on theresources used by the base station for signalling the instruction toinitiate the measurement and reporting process.

In accordance with the approach of FIG. 16 there is a delay betweenmaking channel condition measurements and reporting the informationderived from the channel condition measurements to the base station. Forexample, a terminal device will typically need some time to retune itstransceiver between making measurements. Thus, while step S3 in FIG. 16might be made in association with reference symbols transmitted in agiven subframe, there may then be a delay of one or two subframes whilethe terminal device retunes its transceiver to VC 2 (step S4 in FIG. 16)before the measurements of VC 2 channel conditions (step S5 FIG. 16) canbe made in a subsequent subframe. Because of this the measurements forall four virtual carriers in the example of FIG. 16 might in someembodiments take on the order of 10 subframes to perform. Where channelconditions are rapidly changing this delay might mean the channelcondition information is reported too slowly for effective linkadaptation. However, the inventors have recognised that categories ofterminal device which are likely to be associated with virtual carrieroperation will often be low mobility devices in semi-fixed installationsfor which fast changing channel conditions are less likely to be aconcern.

Because in many virtual carrier implementations it is expected that fastchannel condition reporting is not required (because the channelconditions will be more stable than for conventional devices which willtypically have higher mobility), the inventors have also recognised thathigher layer reporting of channel condition information might be adoptedin accordance with embodiments of the invention. Conventional CQI-typereporting is based on layer one (physical layer) signalling exchange.However, in accordance with embodiments of the invention, theinformation communicated in step S11 FIG. 16 might be communicated withhigher layer signalling, for example in RRC or MAC signalling. Forexample, the communication of information derived from the measurementsof channel conditions may be achieved in an RRC message comprising anewly-defined information element comprising various fields forconveying the information according to the implementation at hand. Forexample, the information element may comprise fields for indicating aterminal-device selected preferred VC frequency location (index) andassociated channel condition measurement and/or fields for indicating atleast a subset of indications of the channel condition measurements forcorresponding virtual carrier locations. As noted above, the virtualcarrier locations may be defined in terms of specific frequencies, or interms of indexing associated with a number of predefined frequencylocations. In another example, corresponding information might beintroduced in a MAC header.

One advantage of using higher-layer (i.e. higher than the physicallayer) signalling for indicating the information derived from thechannel condition measurements in accordance with some embodiments isthe potential for higher transmission reliability because conventionalretransmission protocols, for example based on acknowledgementsignalling, can be adopted. Another advantage in accordance with someembodiments is a reduction in physical layer signalling, with acorresponding reduction in interference, as compared to what mightotherwise be the case if a potentially large number of terminal deviceswere all reporting channel conditions using layer 1 (L1) reporting.

As noted above, it is expected that fast changing channel conditionswill often be less of a concern for the types of terminal devicestypically expected to be operating on a virtual carrier such that thedelays associated with reporting using higher layer signalling asdescribed above are not problematic.

Nonetheless, it is recognised that in some circumstances it may bepreferable for faster reporting of channel conditions for differentpotential virtual carrier frequency locations than might be achievedwith the approach represented in FIG. 16. With this in mind, FIG. 17schematically represents an alternative approach for channel conditionreporting in accordance with another embodiment of the invention.

FIG. 17 is a signalling ladder diagram representing communicationsbetween a base station and a terminal device and steps performed by thebase station and terminal device in accordance with certain embodimentsof the invention. In this example it is assumed the base station and theterminal device again correspond with a base station 1504 and a terminaldevice 1508 such as schematically represented in FIG. 15 with theterminal device 1508 initially attached to the base station usingvirtual carrier VC 1.

As with the example represented in FIG. 16, the signalling representedin FIG. 17 starts from a point at which the base station 1504 hasdetermined that the terminal device should measure channel conditions inaccordance with an embodiment of the invention.

In a first step T1 represented in FIG. 17 the base station 1504 providesthe terminal device 1508 with instructions to undertake the desiredchannel condition measurements. This step is similar to, and will beunderstood from, the corresponding step S1 in FIG. 16. The configurationinformation in this example may include an indication of an order inwhich the base station would like the terminal device to measure channelconditions for an indicated number of different potential virtualcarrier frequency locations.

In a second step T2, the base station provides the terminal device 1508with an indication of an uplink resource allocation on virtual carrierVC 1. This may be provided in accordance with broadly conventionaltechniques, for example for allocating resources for channel conditionreporting on a physical uplink shared channel (PUSCH).

In a third step T3, the terminal device measures channel conditions onVC 1. This step is similar to and will be understood from thecorresponding step S3 in FIG. 16.

In a fourth step T4, the terminal device sends a channel conditionreport to the base station using the uplink resources allocated in stepT2. This reporting may be made in a manner which corresponds closelywith conventional aperiodic CQI reporting in LTE-type telecommunicationsnetworks, for example.

In step T5 the base station determines whether the channel conditions onVC 1 are acceptable. If in step T5 the base station determines from thereported channel conditions for VC 1 that the channel performance isadequate for current scheduling needs, the processing represented inFIG. 17 may stop and the terminal device might simply continue to beserved on virtual carrier VC 1 in the usual manner However, in thisexample it is assumed the base station determines that the reportedchannel conditions for VC 1 are not sufficiently good and the terminaldevice should continue with measuring channel conditions for othervirtual carriers to see if they are better. The decision as to what isconsidered to be sufficiently good will depend on the implementation athand, for example taking account of upcoming scheduling requirements forthe terminal device, and in particular how well these upcomingscheduling requirements would be served on a channel having the measuredchannel conditions. Having decided that the channel conditions on thevirtual carrier VC 1 are not sufficiently good enough to provide adesired level of performance, or are only marginally good enough suchthat significantly better performance might be obtained on a differentvirtual carrier operating at a different frequency location, the basestation communicates in step T6 an indication of an uplink resourceallocation on the next virtual carrier to be measured, in this examplevirtual carrier VC 2. This allocation is sent on the virtual carrier VC1 to which the terminal device is currently tuned.

On receiving the allocation of uplink resources on virtual carrier VC 2,the terminal device interprets this as an indication that it shouldproceed to measure channel conditions on VC 2, and, as schematicallyrepresented in steps T7 and T8, the terminal device tunes is transceiverto the frequency location of virtual carrier VC 2 and measures channelconditions on this channel. These steps T7 and T8 may, for example, beperformed in generally the same manner as steps S4 and S5 represented inFIG. 16 and discussed above.

In step T9, in a manner similar to step T4 performed for VC 1, theterminal device sends a channel condition report derived from themeasurement of channel conditions made on VC 2 to the base station usingthe uplink resources allocated in step T6.

In step T10 the base station determines whether the channel conditionson VC 2 are acceptable for supporting the needs of the terminal devicewith a desired level of performance In this example it is assumed thebase station determines from the reported channel conditions for VC 2that the channel performance would not be sufficiently good, for examplebecause the channel conditions on VC 2 are worse than VC 1, or onlymarginally better.

Thus, in a manner similar to step T6, in step T11 the base stationcommunicates an indication of an uplink resource allocation on the nextvirtual carrier to be measured, in this example virtual carrier VC 3.This allocation is sent on the virtual carrier VC 2 to which theterminal device is currently tuned.

On receiving the allocation of uplink resources on virtual carrier VC 3,the terminal device interprets this as an indication that it shouldmeasure channel conditions on VC 3, and, as schematically represented insteps T12, T13 and T14, the terminal device proceeds to do this, and toreport back to the base station. Steps T12, T13 and T14 performed for VC3 are similar to, and will be understood from, steps T7, T8 and T9performed for VC 2 in the previous iteration.

In step T15 the base station determines whether the channel conditionson VC 3 are acceptable for supporting the needs of the terminal devicewith a desired level of performance In this example it is assumed thebase station determines from the reported channel conditions for VC 3that the channel performance would not be sufficiently good, for examplebecause the channel conditions on VC 3 are worse than VC 1, or onlymarginally better.

Further iterations of steps corresponding to steps T11 to T15 arerepeated for so long as the base station continues to determine thesequentially reported channel conditions for the different virtualcarriers do not merit moving the mobile station to a new virtual carrierfrequency location (or until all potential frequency locations have beenconsidered). The processing represented in FIG. 17 continues from apoint after which a number of these iterations have been performed andthe base station has just determined from the reported channelconditions for virtual carrier VC N-1 that the channel performance forthis virtual carrier would again not be sufficiently good, for examplebecause the channel conditions on VC N-1 are worse than VC 1, or onlymarginally better.

Thus, in a manner similar to steps T6 and T11, in step T16 the basestation communicates an indication of an uplink resource allocation onthe next virtual carrier to be measured, in this example virtual carrierVC N. This allocation is sent on the virtual carrier VC N-1 to which theterminal device is currently tuned.

As with steps T7 to T9 and steps T11 to T14, the mobile station proceedsto tune it is transceiver to virtual carrier VC N (step T17), to measurechannel conditions on a virtual carrier VC N (step T18) and report onthese channel conditions to the base station (step T19).

In step T20, in a manner corresponding to steps to T5, T10 and T15, thebase station determines whether the channel conditions on VC N areacceptable for supporting the needs of the terminal device with adesired level of performance In this example it is assumed the basestation determines the reported channel conditions for VC N aresufficient and that it would be advantageous to move the terminal deviceto a virtual carrier VC N. As noted above, the threshold for the channelconditions at which it is considered advantageous to move a terminaldevice to a new virtual carrier will depend on the implementation athand. In some cases the base station may take account of the upcomingscheduling requirements for the terminal device and decide to move theterminal device to a new virtual carrier if it is determined that thenew virtual carrier can support these requirements while meeting apre-defined performance requirement. In some other cases, the basestation may simply be configured to move the terminal device to a newvirtual carrier if the new virtual carrier is associated with measuredchannel conditions which are better than channel conditions for acurrently used virtual carrier by a certain threshold amount.

Having determined it would be advantageous for the terminal device to ineffect be moved from virtual carrier VC 1 to VCN, the base station maycommunicate this to the terminal device. In the example of FIG. 17 thisis in effect communicated to the mobile station in an implicit manner byvirtue of the fact that the base station does not send to the terminaldevice an allocation of uplink resources on a different virtual carrierto be used by the terminal device for reporting measure channelconditions on that virtual carrier. Instead, in accordance with theapproach of FIG. 17, the base station simply proceeds in a step T21 toprovide the terminal device with a downlink resource allocation on VC Nin accordance with conventional techniques for downlink resourceallocation (for example with signalling on a physical downlink controlchannel, PDCCH, associated with the virtual carrier VC N to which theterminal device is currently tuned). Furthermore, in a step T22 the basestation communicates corresponding user plane data to the mobile stationon the virtual carrier VC N (for example with signalling on a physicaldownlink shared channel, PDSCH, associated with the virtual carrier VC Nto which the terminal device is currently tuned). Thus steps T21 and T22represent normal virtual carrier downlink operation on virtual carrierVC N such that the terminal device has in effect been moved from virtualcarrier VC 1 to virtual carrier VC N as a consequence of the basestation determining this provides an operating improvement for theterminal device as a consequence of the improved channel conditions onVC N.

In principle the initial virtual carrier VC 1 might transpire to be thebest virtual carrier available. In this case, the terminal device mightiterate through all potential virtual carrier frequency locations asdescribed above without the base station determining that any of the newvirtual carrier frequency locations represents an improvement over VC 1.In this case, once the alternate virtual carriers have been considered,or once a subset of the alternate virtual carriers have been considered,without a virtual carrier with improved channel conditions being found,the base station may simply instruct the terminal device to retune to VC1 (or the terminal device might be configured to automatically retune toVC 1 after measuring all other virtual carrier frequency locations) forcontinued operation on the virtual carrier VC 1.

Thus, FIG. 17 represents another approach by which a reduced-capabilityterminal device can estimate channel conditions for a range of differentfrequency locations to aid a base station in determining whether or notthe terminal device should be moved from one virtual carrier frequencylocation to another virtual carrier frequency location.

It will again be appreciated that there are various modifications to theapproach represented in FIG. 17 can be made in accordance with otherembodiments of the invention.

For example, FIG. 17 schematically represents an embodiment in whichuplink resources are sequentially allocated for each channel conditionreport (in steps T2, T6, T11 and T16). Each measure and report in thisapproach to some extent mirrors conventional techniques for reportingindividual channel condition measurements on a physical uplink sharedchannel (PUSCH) in an LTE-type wireless telecommunications network.However, LTE-type wireless telecommunications networks also supportchannel condition reporting on a physical uplink control channel(PUCCH), and other embodiments of the invention may mirror this approachfor reporting the individual measurement conditions. Thus, in someexample embodiments uplink resources for reporting the informationderived from the individual channel condition measurements for eachvirtual carrier might be communicated on resources associated with aphysical uplink control channel (PUCCH) which are retained while aterminal device is connected. In this case there is no need for the basestation to provide individual uplink resource allocations for eachmeasurement of channel conditions. That is to say, in accordance withsome embodiments there may be not be any steps corresponding to stepsT6, T11 and T16 represented in FIG. 17. Instead, uplink resources forreporting on the individual channel condition measurements may bepreviously reserved in accordance with the general techniques of channelcondition reporting on PUCCH. In these cases there may be differenttechniques to allow the terminal device to determine whether it shouldmove onto measure the next channel conditions. For example, whereas inFIG. 17 the terminal device determines that it should retune and makeanother measure channel conditions based on receiving an uplinkallocation message such as T6, in other examples the terminal device maybe configured to simply continue measuring channel conditions ondifferent virtual acid sequence until it receives a downlink resourceallocation corresponding to step T1 in FIG. 17.

In accordance with some embodiments the order in which the virtualcarrier channel conditions are measured may be different. For example ifvirtual carriers are relatively closely spaced, and the base stationrecognises that one virtual carrier has poor channel conditions, thebase station might instruct the terminal device to retune to a virtualcarrier which is not adjacent in frequency.

Furthermore, and this also applies for the embodiment represented inFIG. 16, in accordance with some examples a terminal device might beconfigured to measure channel conditions for a range of frequencylocations which are not necessarily associated with pre-defined virtualcarrier locations. For example, the mobile terminal might measurechannel conditions at various frequency locations throughout an overalloperating bandwidth for the wireless telecommunications system.Predicted channel conditions at other locations, for examplecorresponding to potential virtual carrier frequency locations, can begenerated with interpolation/extrapolation techniques.

It will be appreciated that various modifications can be made to theembodiments described above without departing from the scope of thepresent invention as defined in the appended claims.

Furthermore, although embodiments of the invention have been describedwith reference to an LTE mobile radio network, it will be appreciatedthat the present invention can be applied to other forms of network suchas GSM, 3G/UMTS, CDMA2000, etc. The term MTC terminal as used herein canbe replaced with user equipment (UE), mobile communications device,terminal device etc. Furthermore, although the term base station hasbeen used interchangeably with eNodeB it should be understood that thereis no difference in functionality between these network entities.

Thus, apparatus and methods for providing feedback on channel conditionsin a wireless telecommunications system are described. The wirelesstelecommunications system comprises a base station arranged tocommunicate with a plurality of terminals device using frequenciesspanning a system frequency bandwidth, wherein at least one terminaldevice is a reduced capability terminal device comprising a tuneabletransceiver configured to receive downlink transmissions from the basestation using only a restricted frequency bandwidth which is smallerthan and within the system frequency bandwidth. The base stationtransmits configuration signalling to the reduced capability terminaldevice to indicate the reduced capability terminal device should measurechannel conditions for different frequencies. The terminal deviceresponds by sequentially tuning its transceiver to different frequencylocations for the restricted frequency bandwidth within the systemfrequency bandwidth and making measurements of channel conditions at thedifferent frequency locations to provide a corresponding plurality ofmeasurements of channel conditions. The reduced capability terminaldevice is configured to communicate information derived from themeasurements of channel conditions to the base station. The informationmay comprise an indication of the measured channel conditions for thedifferent frequency locations, or an indication of one or more frequencylocations for which the corresponding measurement of channel conditionsmeet a pre-defined selection criterion. The base station subsequentlyschedules downlink transmissions for the terminal device in a mannerthat takes account of the information received from the terminal device.

Further particular and preferred aspects of the present invention areset out in the accompanying independent and dependent claims. It will beappreciated that features of the dependent claims may be combined withfeatures of the independent claims in combinations other than thoseexplicitly set out in the claims.

REFERENCES

-   [1] ETSI TS 122 368 V10.530 (July 2011)/3GPP TS 22.368 version    10.5.0 (Release 10)-   [2] UK patent application GB 1101970.0-   [3] UK patent application GB 1101981.7-   [4] UK patent application GB 1101966.8-   [5] UK patent application GB 1101983.3-   [6] UK patent application GB 1101853.8-   [7] UK patent application GB 1101982.5-   [8] UK patent application GB 1101980.9-   [9] UK patent application GB 1101972.6-   [10] UK patent application GB 1121767.6-   [11] UK patent application GB 1121766.8-   [12] Holma H. and Toskala A, “LTE for UMTS OFDMA and SC-FDMA based    radio access”, John Wiley and Sons, 2009

1. A method of operating a terminal device in a wirelesstelecommunications system comprising a base station arranged tocommunicate with a plurality of terminal devices using frequenciesspanning a system frequency bandwidth, wherein the terminal devicecomprises a tuneable transceiver configured to receive downlinktransmissions from the base station using a restricted frequencybandwidth which is smaller than and within the system frequencybandwidth, and wherein the method comprises: sequentially tuning thetransceiver to different frequency locations for the restrictedfrequency bandwidth within the system frequency bandwidth and makingmeasurements of channel conditions at the different frequency locationsto provide a corresponding plurality of measurements of channelconditions; and communicating information derived from the plurality ofmeasurements of channel conditions to the base station.
 2. The method ofclaim 1, wherein the different frequency locations for the restrictedfrequency bandwidth within the system frequency bandwidth comprise aplurality of predefined frequency locations.
 3. The method of claim 1,wherein the measurements of channel conditions comprise measurements ofnoise and/or measurements of interference.
 4. The method of claim 1,wherein the information communicated to the base station comprises anindication of at least a subset of the plurality of measurements ofchannel conditions.
 5. The method of claim 1, further comprising theterminal device selecting a preferred frequency location for therestricted frequency bandwidth within the system frequency bandwidthbased on the plurality of measurements of channel conditions.
 6. Themethod of claim 5, wherein the information communicated to the basestation comprises an indication of the preferred frequency location forthe restricted frequency bandwidth within the system frequencybandwidth.
 7. The method of claim 6, wherein the informationcommunicated to the base station further comprises an indication of ameasurement of channel conditions for the preferred frequency locationfor the restricted frequency bandwidth within the system frequencybandwidth.
 8. The method of claim 1, wherein the informationcommunicated to the base station comprises an indication of locationsfor the restricted frequency bandwidth within the system frequencybandwidth associated with measurements of channel conditions meeting apredefined criterion.
 9. The method of claim 1, wherein the informationcommunicated to the base station comprises an indication of one or moretimes associated with the plurality of measurements of channelconditions.
 10. The method of claim 1, wherein the downlink radio framestructure for the wireless telecommunications system comprises a seriesof time slots and the measurements of channel conditions for differentfrequency locations are made in different time slots.
 11. The method ofclaim 10, wherein consecutive measurements of channel conditions fordifferent frequency locations are made in non-consecutive time slots.12. The method of claim 1, wherein the information derived from theplurality of measurements of channel conditions is communicated to thebase station with signalling on a layer which is higher than a physicallayer.
 13. The method of claim 12, wherein the information derived fromthe plurality of measurements of channel conditions is communicated tothe base station with radio resource control, RRC, signalling.
 14. Themethod of claim 1, wherein the information derived from the plurality ofmeasurements of channel conditions is communicated to the base stationwith physical layer signalling.
 15. The method of claim 1, wherein theinformation communicated to the base station is sent in a plurality ofchannel condition reports corresponding with respective one of theplurality of measurements of channel conditions.
 16. The method of claim15, further comprising receiving a plurality of resource allocationmessages from the base station indicating uplink transmission resourcesto be used for sending respective ones of the channel condition reports.17. The method of claim 16, wherein different ones of the plurality ofresource allocation messages are received with the transceiver tuned todifferent frequency locations for the restricted frequency bandwidthwithin the system frequency bandwidth.
 18. The method of claim 1,further comprising receiving from the base station an indication of thedifferent frequency locations for the restricted frequency bandwidthwithin the system frequency bandwidth at which to make the plurality ofmeasurements of channel conditions
 19. The method of claim 1, furthercomprising receiving an indication from the base station that theterminal device's transceiver should be retuned to a frequency locationselected by the base station based on the information received from theterminal device.
 20. A terminal device for use in a wirelesstelecommunications system comprising a base station arranged tocommunicate with a plurality of terminal devices using frequenciesspanning a system frequency bandwidth, wherein the terminal devicecomprises a tuneable transceiver configured to receive downlinktransmissions from the base station using a restricted frequencybandwidth which is smaller than and within the system frequencybandwidth, and wherein the terminal device is configured to:sequentially tune the transceiver to different frequency locations forthe restricted frequency bandwidth within the system frequency bandwidthand make measurements of channel conditions at the different frequencylocations to provide a corresponding plurality of measurements ofchannel conditions; and to communicate information derived from theplurality of measurements of channel conditions to the base station. 21.The terminal device of claim 20, wherein the different frequencylocations for the restricted frequency bandwidth within the systemfrequency bandwidth comprise a plurality of predefined frequencylocations.
 22. The terminal device of claim 20, wherein the measurementsof channel conditions comprise measurements of noise and/or measurementsof interference.
 23. The terminal device of claim 20, wherein theinformation communicated to the base station comprises an indication ofat least a subset of the plurality of measurements of channelconditions.
 24. The terminal device of claim 20, wherein the terminaldevice is further configured to select a preferred frequency locationfor the restricted frequency bandwidth within the system frequencybandwidth based on the plurality of measurements of channel conditions.25. The terminal device of claim 24, wherein the informationcommunicated to the base station comprises an indication of thepreferred frequency location for the restricted frequency bandwidthwithin the system frequency bandwidth.
 26. The terminal device of claim25, wherein the information communicated to the base station furthercomprises an indication of a measurement of channel conditions for thepreferred frequency location for the restricted frequency bandwidthwithin the system frequency bandwidth.
 27. The terminal device of claim20, wherein the information communicated to the base station comprisesan indication of locations for the restricted frequency bandwidth withinthe system frequency bandwidth associated with measurements of channelconditions meeting a predefined criterion.
 28. The terminal device ofclaim 20, wherein the information communicated to the base stationcomprises an indication of one or more times associated with theplurality of measurements of channel conditions.
 29. The terminal deviceof claim 20, wherein the downlink radio frame structure for the wirelesstelecommunications system comprises a series of time slots and thewherein the terminal device is configured such that the measurements ofchannel conditions for different frequency locations are made indifferent time slots.
 30. The terminal device of claim 29, wherein theterminal device is configured such that consecutive measurements ofchannel conditions for different frequency locations are made innon-consecutive time slots.
 31. The terminal device of claim 20, whereinthe terminal device is configured such that the information derived fromthe plurality of measurements of channel conditions is communicated tothe base station with signalling on a layer which is higher than aphysical layer.
 32. The terminal device of claim 31, wherein theterminal device is configured such that the information derived from theplurality of measurements of channel conditions is communicated to thebase station with radio resource control, RRC, signalling.
 33. Theterminal device of claim 20, wherein the terminal device is configuredsuch that the information derived from the plurality of measurements ofchannel conditions is communicated to the base station with physicallayer signalling.
 34. The terminal device of claim 20, wherein theterminal device is configured such that the information communicated tothe base station is sent in a plurality of channel condition reportscorresponding with respective one of the plurality of measurements ofchannel conditions.
 35. The terminal device of claim 34, wherein theterminal device is configured to receive a plurality of resourceallocation messages from the base station indicating uplink transmissionresources to be used for sending respective ones of the channelcondition reports.
 36. The terminal device of claim 35, wherein theterminal device is configured such that different ones of the pluralityof resource allocation messages are received with the transceiver tunedto different frequency locations for the restricted frequency bandwidthwithin the system frequency bandwidth.
 37. The terminal device of claim20, wherein the terminal device is configured to receive from the basestation an indication of the different frequency locations for therestricted frequency bandwidth within the system frequency bandwidth atwhich to make the plurality of measurements of channel conditions 38.The terminal device of claim 20, wherein the terminal device isconfigured to receive an indication from the base station that theterminal device's transceiver should be retuned to a frequency locationselected by the base station based on the information received from theterminal device.